For ternary heteroepitaxial layers, the independent determination of the composition and state of strain requires x-ray rocking curve measurements for at least two different hkl reflections because the relaxed lattice constant is a function of the composition. The usual approach involves the use of one symmetric reflection and one asymmetric reflection. Two rocking curves are measured at opposing azimuths for each hkl reflection. Thus, it is possible to account for tilting of the hkl planes in the epitaxial layer with respect to the hkl planes in the substrate, by averaging the peak separations obtained at the opposing azimuths. This procedure presents a practical problem in the case of asymmetric reflections, for which the tilting can only be canceled if the rocking curve for one azimuth is obtained using theta-phi incidence. A preferable approach, which provides sharper, more intense rocking curves and greater experimental accuracy, is to measure both asymmetric rocking curves at theta+phi incidence. This approach requires that the data be corrected for the tilting of the asymmetric planes introduced by tetragonal distortion. Here we have presented a new analytic procedure that incorporates the tilting of asymmetric diffracting planes due to tetragonal distortion. The new procedure allows the measurement of all rocking curves at theta+phi incidence. We have applied this new method to the case of ZnSySe1-y grown heteroepitaxially on GaAs (001), using 004 and 044 x-ray rocking curves. We have shown that neglect of the tilting in asymmetric planes results in gross errors in the calculated values of composition (as much as 35 times) and in-plane strain (as much as 2.6 times) for this material.