The microwave spectra of the 13C-species of CF3Br and CF3I have been measured with the aid of radiofrequency pumping of the quadrupole sub-levels. In addition, rotational data for the main species have been refined. The following zero-point average structures (rz) have been determined: CF3Br: C—F = 1.3265(23), C—Br = 1.9234(31)Å, ∠FCF = 108.81(25)°, CF3I: C—F = 1.3285(23), C—I = 2.1438(27)Å, ∠FCF = 108.42(23)°. Particular attention has been paid to the effect of isotopic variation in bond length in the structural calculations. The structures agree well with those recently obtained by combining electron diffraction and microwave data. Accurate Stark measurements have been performed for a number of transitions including the J= 3 → 4, K= 3, F= 7/2 → 9/2 transition of CF379Br and the J= 4 → 5, K= 3, F= 3/2 → 5/2 transition of CF3I. The results have been fitted using a diagonalization treatment of the quadrupole-Stark matrix, set up in the uncoupled basis. The dipole moment of CF3Br was determined to be 0.639(2) D and that of CF3I, 1.048(3) D. The structural properties of CF3Br and CF3I are shown to fit some trends with related molecules.