In this paper, the sedimentology, petrography and biostratigraphy of the Nam Phong Formation and the overlying Mesozoic Khorat Group in NE Thailand are described. Data were gathered during extensive studies in the field (about 100 outcrop localities were investigated) and from laboratory analyses of over 500 surface samples. The Nam Phong Formation is dated accurately for the first time as late Norian to Rhaetian, while the Phra Wihan, Sao Khua, Phu Phan and Khok Kruat Formations (of the Khorat Group) are reassigned to the Early Cretaceous (Berriasian to Aptian) on the basis of new palynological data. The age of the intervening Phu Kradung Formation, which is conventionally assigned to the Lower Jurassic, is probably Late Jurassic or Early Cretaceous, although this cannot be determined with confidence. Changes in provenance between the Nam Phong Formation and the overlying Phu Kradung Formation suggest a possible sedimentary hiatus. Palaeocurrent data suggest that sediments were mainly sourced from the north and east. Part I of this paper describes the sedimentology and biostratigraphy of these units; Part 2, which is scheduled to appear in the April, 1996 issue of this Journal, reports on their diagenesis and reservoir quality.