Title: Speech-on-speech masking: Properties of the masker speech that change its effectiveness. Abstract: In this lecture, I will present two data sets evaluating sentence recognition in the presence of competing speech maskers. The importance of who is talking in the background and what they are saying will be evaluated. In the first set of experiments we will assess if one of the two talkers within the masker speech dominates the masker’s overall effectiveness. In the second set of experiments, we will explore whether the semantic meaning of the masker speech matters when controlling for syntax, lexical content, and the talker’s voice.