The "Decision-Making Questionnaire" (DMQ) was developed and validated in order to examine the factors that affect decision making. The investigation was carried out with two samples, one of 170 participants and the other of 425 of both sexes. Each sample was divided into three age ranges: young students (18-25 years), adults (26-60 years), and seniors (61-75 years). Psychometric analyses revealed satisfactory internal consistency and a first-order factor structure comprising 10 scales: Uncertainty, Time/money pressure, Information and goals, Consequences of the decision, Motivation, Self-regulation, Cognition, Emotion, Social pressure, and Work pressure. These scales in turn yielded a second-order factor structure of Task, Subject, and Context. The results highlight the ways in which youths, adults, and seniors are influenced by diverse factors when making decisions. They also suggest that men are more likely than women to seek and analyse information, whereas women place higher value on the available time and money. The article concludes that the DMQ examines the factors that have the most influence on professional decisions and it could guide adolescents in the vocational decision process. (Contains 3 tables and 1 figure.)