Thirteen of the 52 independent third-order anharmonic potential constants have been determined from the vibration-rotation constants, 21 each obtained for 12CH 2F 2 and 12CD 2F 2. Precision of the derived third-order constants is not high except for f 333, where three means one of the CF stretching coordinates. However, the observed vibration-rotation constants were reproduced by these constants, with average deviations of 20.51, 11.25, and 13.83 MHz for α A , α B , and α C . Anharmonic potential constants thus derived were used to calculate the vibration-rotation constants of 13CH 2F 2, 13CD 2F 2, and 12CDHF 2, except the terms containing the f 111 constant (superscript 1 means one of the CH stretching coordinates). The ground-state rotational constants of five isotopic species, which were corrected for the vibration-rotation constants thus calculated, were subjected to a least-squares analysis and led to the equilibrium structure, CH = 1.084 ± 0.003 Å, CF = 1.3508 ± 0.0005 Å, HCH = 112.8 ± 0.3°, and FCF = 108.49° ± 0.06°, and to f 111 = −37 ± 7 md/ A ̊ 2 . The r z structure was also calculated for 12CH 2F 2 and 12CD 2F 2, and was compared with the r e structure. The isotope effect, r z (CH)- r z (CD), was found to be 0.0025 Å.