Impaired control occupies a focal position in the medical model perspective of pathological gambling and is central to the concept of addictive processes generally. Nonetheless there have been few studies on self-control in gambling behaviour. A questionnaire was developed to measure subjectively assessed self-control in poker machine players. This Self-Control Scale comprised a framework adopted from Cameron (1985) distinguishing between (i) the likelihood of starting a session and continuing a session, and (ii) internal and external determinants. The Scale and a descriptive questionnaire on demographics and actual gambling behaviour was administered to 26 problem gamblers and 40 players of different levels of involvement. The results indicated that the Scale had acceptable internal consistency and validity. The Scale effectively discriminated between problem and non-problem players. The differences on specific items between low-frequency, high-frequency and problem groups were incorporated into a hypothetical developmental progression. Negative emotions such as frustration and depression and the belief in chasing (Lesieur, 1979, 1984) were perceived to be the most significant determinants of impaired control problem gamblers.