The techniques of attenuated total reflection (ATR) and grating-coupling are well established as powerful experimental probes for surface polaritons. Even more versatile is the combination of these methods into a single technique: ATR grating-coupling. A theoretical account of ATR grating-coupling is given within the limitations of first-order perturbation theory. The layout of the system studied here is essentially that of the ATR arrangement due to Otto (Otto A 1968 Z. Phys. 216 398) for probing surface polaritons in a composite thin film. The key additional feature here is the presence of a classical grating on the base of the coupler prism. The composite thin film consists of spherical grains embedded in a host medium, both constituents being surface-active and each characterized by a specific frequency-dependent dielectric function. The dispersion of surface polaritons propagating along the interfaces of the composite film show a strong dependence on the relative volume content of the constituents of the film. In particular, the polariton structures occurring in the reflectivity spectra shift to higher frequencies as the fractional volume content of the spherical grains is increased.