Most infant social referencing studies have assumed that infants would be more likely to engage in social looking and be influenced by adults' message when a context is ambiguous. The present study empirically tested the effect of stimulus ambiguity on infants' referencing behaviours, with three different stimuli (positive, ambiguous, and negative), two different messages (happy and fearful), two different message providers (mother and stranger), and in two age groups (12 and 16 month olds). A typical social referencing paradigm was used and infants' social looking and regulation were measured. Infants looked at adults more frequently and faster during ambiguous situations than during unambiguous situations. They also tended to regulate their affect and behaviour based on adults' message only towards ambiguous toys. Older infants tended to look at adults faster, and showed stronger reactions towards ambiguous stimuli than younger infants, suggesting that infants' social development may moderate the effect of stimulus ambiguity on social referencing. Overall, results indicated that the ambiguity postulate is a legitimate assumption for infant social referencing. Copyright © 2010 John Wiley & Sons, Ltd.