We have investigated the possibility of determining the relative concentrations of two radon isotopes ( 222Rn and 220Rn) in the air, using solid state nuclear track detectors (CR-39) as alpha spectrometers. The detectors were exposed to 222Rn and its daughters and 220Rn and its daughters in the air. Analyzing only roundish tracks, it was observed that the performance of CR-39 as alpha spectrometer varies with etching time, improving markedly for long etching times.1821139147