The molecular structure of SR-90 and CR-39 is very similar except that the average length of carbonate linkages in SR-90 is 1.6 times longer than in CR-39. However, the effect of vacuum and carbon dioxide (CO 2 ) on the track registration of these two detectors is very different: 1) the sensitivity of CR-39 becomes very low in vacuum; the sensitivity of SR-90 does not go down to zero even after storage in vacuum for a long time; 2) the sensitivity of SR-90 decreases both in air and in vacuum but the original sensitivity can be restored by storing it in CO 2 of 1 3 atm for several days. We also found that the sensitization of latent tracks by oxygen (O 2 ) is effective only at the time of irradiation but the sensitization by CO 2 is effective after irradiation.