Here we present a new quantitative model for the interpretation of the asymmetric broadening of the full width at half maximum (FWHM) of X-ray rocking curves due to different dislocation densities on different slip systems. Our analysis indicates that an asymmetry in densities of dislocations on different slip systems will cause the FWHM of the rocking curve to vary with the azimuth, for a symmetric reflection. To illustrate this method we have applied it to heteroepitaxial ZnSe on GaAs (0 0 1) using a Bartels high-resolution diffractometer and the symmetric 004 reflection. 004 rocking curves were measured at azimuths spaced by 20 degrees. The FWHM was maximum at [1 (1) over bar 0] and [(1) over bar 1 0] azimuths but minimum with [1 1 0] and [(1) over bar(1) over bar0] azimuths. This indicates that there are fewer dislocations on the slip systems having misfit dislocation segments with [1 (1) over bar 0] type line vectors than on those with [1 1 0] type line vectors in this system. We refer to these dislocation densities as D-A and D-B, respectively. Using this experimental data and the present analytical model, the densities of dislocations on the two types of slip systems were found to be D-A=2.0 x 10(9) cm(-3) and D-B=2.5 x 10(9) cm(-3). This difference in dislocation densities could be due to the difference in mobility for alpha and beta type dislocations. (C) 2010 Elsevier B.V. All rights reserved.