Experiments on collisionally assisted two-photon absorption (TPA) in calcium vapour buffered by argon gas have been performed using two tunable dye lasers. Both direct TPA and stepwise transitions through the collisionally populated resonance level. 4s4p ¹P₁, have been observed. Detailed study of the lineshapes of the stepwise transitions to the 4p² ¹S₀, 4p² ¹D₂ and 4s6s ¹S₀ levels reveals large differences in the strengths of the Ca-Ar interatomic potentials for these levels. These differences probably arise from configuration mixing.