Objectives: Recent research has demonstrated that pupil dilation, a measure of mental effort (cognitive processing load), is sensitive to differences in speech intelligibility. The present study extends this outcome by examining the effects of masker type and age on the speech reception threshold (SRT) and mental effort. Design: In young and middle-aged adults, pupil dilation was measured while they performed an SRT task, in which spoken sentences were presented in stationary noise, fluctuating noise, or together with a singletalker masker. The masker levels were adjusted to achieve 50% or 84% sentence intelligibility. Results: The results show better SRTs for fluctuating noise and a single-talker masker compared with stationary noise, which replicates results of previous studies. The peak pupil dilation, reflecting mental effort, was larger in the single-interfering speaker condition compared with the other masker conditions. Remarkably, in contrast to the thresholds, no differences in peak dilation were observed between fluctuating noise and stationary noise. This effect was independent of the intelligibility level and age. Conclusions: To maintain similar intelligibility levels, participants needed more mental effort for speech perception in the presence of a single-talker masker and then with the two other types of maskers. This suggests an additive interfering effect of speech information from the single-talker masker. The dissociation between these performance and mental effort measures underlines the importance of including measurements of pupil dilation as an independent index of mental effort during speech processing in different types of noisy environments and at different intelligibility levels. Copyright © 2012 by Lippincott Williams &Wilkins.