Triage assessment of patients on arrival at emergency departments involves complex decision making, resulting in categories being assigned to prioritize patients' needs for attention. The actual process of triage decision making has received limited attention. The aims of this study were to describe aspects of the triage decision-making process used by both more and less experienced nurses (n = 20) and to test the effect of uncertainty in the triage situation on the use of probability judgements (heuristics). Six triage cases based on actual triage situations were simulated to subjects, and their verbal protocols were collected. Protocols were transcribed and analysed. Main findings were: in conditions of higher uncertainty in triage situations all nurses used more probability in their judgements (t-test -2.37, df = 17, P = 0.03) with the heuristic of representativeness being relied on the most. The triage categories finally assigned for each triage situation showed no agreement on a specific triage category for each triage case and past triage experiences were used in decision making. The more experienced group reported higher estimations of correctness regarding the final category assigned, used more single previously experienced cases from memory, collected less data and made more judgements than the less experienced group. Further examination of the decision-making process of triage assessment should occur with attention to the variable use of triage categories, the role of past triage experiences in making judgements and development of triage decision rules for skilling nurses for triage.