The simultaneous effects of ion Larmor radius and collisions with neutral atoms are investigated on Kelvin-Helmholtz and Rayleigh-Taylor configurations. It is found that the Kelvin configuration is stable if both finite Larmor radius effects and the collisions are taken into account for all wave-numbers greater than the critical value U/2ν. When the system is unstable, it is found that, whereas the Larmor radius has a stabilizing influence, a collision frequency has a stabilizing effect when it is small and a destabilizing effect when it exceeds a certain value. In the case of the Rayleigh-Taylor configuration it is found, however, that the collisions have a stabilizing influence.