The objective of this work is the revision of the most conventional methods used to determine the thermal maturity of the Los Monos Formation in the Tarija Basin. For this, the thermal maturity of the formation was determined from multiple vitrinite reflectance analysis, programmed pyrolysis, index of thermal alteration and clay index. Work was done on outcrop samples in the areas of Balapuca and Alarache on the border between Argentina and Bolivia, and well samples in the areas of the Ramos, Tartagal, Vespucio and Tonono hydrocarbon fields in northern Argentina, and Tacobo in the Bolivian Chaco area. The Los Monos Formation is the main source rock of the Paleozoic area of the Subandino of the north of Argentine and south of Bolivia. It is characterized by its low organic content (COT), on average less than 1%. Different divisions were postulated for this unit according to its geochemical characteristics, identifying gas generating sections and oil generating sections. The geochemical studies carried out (Tmax and IP Production Index of programmed pyrolysis) on outcrop samples as in well samples, would indicate that the formation is in the oil window. Regarding the type of kerogen, from the data of programmed pyrolysis (IH and IO), the Los Monos Formation contains type III kerogen which is primary gas generators. In contrast to the geochemical data, from the quantification of the expansive Sm layers in the argilomineral I / S it would indicate that the Los Monos Formation is in a stage of deep diagenesis (telodiagenesis) in the gas generation window. In this way, both the vitrinite reflectance and the IAT show values of maturity related to diagenesis stages lower than those provided by the method of measuring the expansive layers in the interstratified IS. Therefore, it is postulated that the method of measurement of expansive layers is more advantageous in source rocks with low organic content, as happens with the Los Monos Fm, also aligned with what was observed in the wells (presence of gas in the boreholes and in the associated reservoirs such as the Huamampampa Fm). Hence, this method contributes as an alternative to determining thermal maturity to conventional methods, especially in the case of formations with characteristics similar to the Los Monos Fm