Backward masking is a popular method to prevent awareness of facial expressions, but concerns have been expressed as to the effectiveness of masking in previous research, which may have resulted in unjustified claims of unconscious processing. We examined the minimum presentation time for discrimination of fearful, angry, happy and neutral faces in a backward masking task using both objective sensitivity measures, based on signal detection analysis, and subjective awareness ratings. Results from two experiments showed for all expressions the mean sensitivity and the sensitivity scores of most individual participants were above chance at presentation times of 20 ms. Awareness ratings for happy, fearful and anger also exceeded baseline ratings from 20 ms onwards. Overall sensitivity in both experiments was greatest for happy expressions, which is an agreement with previous reports. The results support the possibility of incomplete masking in earlier studies that used masking to prevent awareness of facial expressions.