Application of a sufficiently strong magnetic field Hx along the antiferromagnetic easy axis (x) of the isomorphic, distorted perovskites YFeO3 and YCrO3, results in the smooth rotation of the antiferromagnetic vector in the x-z plane, reaching the z axis at the critical fields Hcr=74±1 kOe and Hcr=40±1 kOe at 4.2°K, for YFeO3 and YCrO3, respectively. A detailed analysis of the magnetization curve mx(Hx) provides an unambiguous determination of the quartic and uniaxial-type anisotropy in the x-z plane of YFeO3 (spin= 52). In agreement with theory, quartic anisotropy terms are not found in YCrO3 (spin= 32). Only in the latter case are the uniaxial anisotropy and the reorientation field calculable from the initial susceptibilities. Comparison of mx(Hx) with mz(Hz) for H>Hcr provides a quantitative separation between the antisymmetric exchange (HD) and single-ion anisotropy (Hxz) contributions to the weak ferromagnetic canting. We find |Hxz/HD| <0.02 for YFeO3 and |Hxz/HD| <0.05 for YCrO3, at 4.2°K.