The performance of an array of gas proportional counters (Model LB4200TM) was characterized for assessing gross radioactivity in drinking water. Among others, the examined parameters included detection efficiency and minimal detectable activity concentration. The study of the system parameters was conducted using water samples with known activities of 230Th and 90Sr and known salinity levels that were prepared according to the co-precipitation and evaporation methods. The detection efficiencies for alpha and beta in water samples were found to be 16% and 37%, respectively. In addition, the extraction efficiencies of radionuclides in water samples using the co-precipitation and evaporation methods were found to be approximately 90%. Minimal detectable concentrations for gross alpha and gross beta were found to be typically 0.02 Bq/L and 0.13 Bq/L, respectively. Results demonstrate that although the system detectors as well as the prepared water samples have relatively small diameters (40–49 mm), the system and methods used meet the criteria for determining gross alpha and gross beta in drinking water.