The arguments of Lewis and Tolman that predict mass dilation are critically reviewed. It is pointed out that their conclusion that two observers in relative motion would measure different relative velocities of a given object in their model collision system is incompatible with Einstein’s second postulate of the special theory of relativity, in particular for the case when the object is a light pulse. It also contradicts assumptions that have been made in succeeding years about the respective measurements of the relative velocities of meta-stable particles of different observers. Momentum can still be conserved in the Lewis-Tolman collision model if one assumes that the in situ masses of the two objects are not the same but differ in such a way as to exactly compensate for any mass dilation effect caused by the
acceleration of one of the observers.