Whole-interval Differential Reinforcement of Other Behaviours (DRO), which reinforces set periods of behaviour absence, has been a widely used treatment in the reduction of inappropriate behaviours. Momentary DRO, a variant of this schedule which reinforces behaviour absence only at specific moments, may offer higher levels of reinforcement for clients and greater ease of administration. The primary aim of this thesis was to investigate the relative effectiveness of momentary DRO and whole-interval DRO in reducing inappropriate behaviour. The thesis also examined the claims that momentary DRO delivers greater reinforcement than whole-interval DRO and that momentary DRO is easier to programme. Experiments 1 to 6 showed momentary DRO to be at least as effective as whole-interval DRO in reducing inappropriate behaviour. There was also evidence to suggest momentary DRO produced a more rapid behaviour reduction and that it was more effective in reducing non-targeted inappropriate behaviours. Experiments I, 6 and 7 showed that momentary DRO consistently delivered higher levels of reinforcement than whole-interval DRO. Contrary to predictions, whole-interval DRO was reported to be as easy to administer as momentary DRO (Experiments 9 and 10). However, as subjects did not programme both schedules this result must be considered provisional. Experiments 9 and 10 showed that reinforcement was delivered more accurately in whole-interval DRO, although there was evidence to suggest that administration of this schedule was more effected by concurrent tasks than momentary DRO. It can therefore be concluded that the effectiveness of momentary DRO has been underestimated. It is clear, in view of its effectiveness and of the greater density of reinforcement it provides, that momentary DRO presents a viable, and in some cases superior, alternative to whole-interval DRO in the reduction of inappropriate behaviour.