The purpose of this study was to 1) characterise word recognition in a speech masker for preschoolers tested using closed-set, forced-choice procedures and 2) better understand the stimulus and listener factors affecting performance.Speech recognition thresholds (SRTs) in a two-talker masker were evaluated using a picture-pointing response with two sets of disyllabic target words. ChEgSS words were previously developed for children ≥5 years of age, and simple words were developed for preschoolers. Familiarisation ensured accurate identification of target words before testing.Participants were 3- and 4-year olds (n = 21) and young adults (n = 10) with normal hearing.Preschoolers and adults had significantly lower SRTs for the simple words than the ChEgSS words, and lower SRTs for early-acquired than later-acquired ChEgSS words. For both word sets, SRTs were approximately 11-dB higher for preschoolers than adults, and child age was associated with SRTs. Preschoolers' receptive vocabulary size predicted performance for ChEgSS words but not simple words.Preschoolers were more susceptible to speech-in-speech masking than adults, with a similar child-adult difference for the ChEgSS and simple words. Effects of receptive vocabulary in preschoolers' recognition of ChEgSS words indicate that vocabulary size is an important consideration, even when using closed-set methods.