State-resolved integral cross sections for the inelastic scattering of the Hund’s case (a) radical NCO((X) over tilde(2)II) by Ar have been measured in a pulsed, crossed-beam apparatus. An extensive comparison has been made between its results and those for NCO+He. Substantial differences were found in nearly every aspect. It was conjectured that the observed "mass effects" arose mainly from the difference in interaction time which changed the dynamics from predominantly an impulsive type for NCO+He to a regime governed by multiple encounter collision for NCO+Ar.