Beta detection efficiency curves are reported for some radionuclides of interest (40K,90Y,90Sr+90Y,137Cs,144Ce+144Pr,147Pm,155Eu and204Tl) contained in sources of a suitable material and having a superficial density ranging from 3 to 110 mg/cm2; the curves were experimentally drawn by measuring the reference sources in different geometries by means of three GM window detectors (one of them with low background) and of a 2π flow proportional counter. These curves have been used for several years in the Radiochemistry Service Laboratories to determine radionuclides in sources of different thickness obtained by specific radiochemical separations from various environmental samples.