Izapa is famous for its monumental architecture and extensive corpus of carved stelae dated to the Late Formative Guillén phase (300–100 cal b.c.). The site was first established, however, as the capital of a kingdom during the second half of the Middle Formative period (750–300 cal b.c.). Little is known of the first centuries of the site's occupation or how this early kingdom coalesced with Izapa as its capital. In 2012, the Izapa Regional Settlement Project (IRSP) excavated 21 test units and ran 10 radiocarbon accelerator mass spectrometry (AMS) dates in order to begin correcting this lacuna. These excavations were the first at the site to screen soil matrices and recover artifact samples that can be quantitatively analyzed.

We undertook excavations in areas north and south of Group B, the original center of Izapa. This work dates the northern expansion of the site's main platform (under Mound 30a) to the Terminal Formative Itstapa phase (cal a.d. 100–300) that resulted in a doubling of the platform's size. Further, we documented that there were three distinct construction episodes in the Terminal Formative expansion and that a central staircase and ramp were built of stone during the second episode. Buried below the Terminal Formative platform expansion was a white clay surface built during the Escalón phase (750–500 cal b.c.) and used through to Guillén times. At the long, linear Mound 62 that defines the eastern edge of Izapa's site core, we documented two episodes of Guillén-phase monumental construction. Buried below this construction fill at Mound 62, a hearth feature and stone alignment are dated to the late Middle Formative based on radiocarbon assays and the results of ceramic analysis. Excavations at Mound 72 and 73 documented that Izapa's E-Group (newly recognized with lidar [light detection and ranging] data) was established in the late Middle Formative period and then significantly augmented during the Guillén phase. The architectural program at Izapa saw its apogee during the Late Formative period, but was first established during the preceding centuries of the Middle Formative. Ten new AMS dates confirm the dating of the Escalón, Frontera, and Guillén phases to 750–100 cal b.c.

Ceramic analysis allowed us to differentiate quantitatively between midden deposits and construction fill through the site's occupation and to recognize domestic versus public spaces during the first centuries of the Izapa kingdom's coalescence. We identify late Middle Formative period middens based on the high density of ceramics in addition to good surface preservation of sherds and a lack of temporal mixing of types. The designation of high-artifact density middens contrasts with the contents of Late and Terminal Formative construction fill with lower ceramic sherd densities and mixing of temporally diagnostic types. Off-mound contexts (where construction fill was mined) had even lower ceramic densities than construction fill and the sherds were very eroded. Analysis of ceramic remains from late Middle Formative period midden deposits also allowed us to infer differences in public and domestic areas of the site during the first centuries of its occupation. Formal and metric variables from these ceramic assemblages identify dish-to-jar ratios that differentiate domestic contexts (with an assortment of vessel forms) from more publically oriented areas of the site (with more serving dishes). The differential distribution of rim diameters of fancy and plain dishes allows us to identify areas of Izapa where domestic activities predominate and indicate that more publically oriented feasting practices occurred at the site center near the main pyramid (Mound 30a) during the late Middle Formative period.