In beta emitters, in order to evaluate the absorbed dose rate at different tissue depths, it is necessary to determine the transmission factors. In this work, the transmission factors determined in beta secondary standard radiation beams are presented. For this purpose, an extrapolation chamber was used. The results obtained were considered acceptable, and they are within the uncertainties in comparison with the values provided by the source calibration certificate. The maximum differences between the results obtained in this work and those from the calibration certificate were 3.3%, 3.8% and 5.9% for 90Sr/90Y, 85Kr and 147Pm sources respectively.Conselho Nacional de Desenvolvimento Cient??fico e Tecnol??gico (CNPq)Funda????o de Amparo ?? Pesquisa do Estado de S??o Paulo (FAPESP)CNPq: 142297/2015-1; 301335/2016-8FAPESP: 08/ 57863-2