The goal was to measure the contribution of semantic and syntactic constraints to the probability of word recognition in sentences. Normally hearing adult Ss listened to sets of four‐word sentences at four S/N ratios. There were three types of sentences: (1) zero predictability, i.e., nonsense word strings; (2) low predictability, i.e., syntactically correct but semantically anomolous sentences; (3) high predictability, i.e., syntactically and semantically correct sentences. Word recognition probabilities were measured for each condition. The contribution of syntax was computed as the ratio of the logarithms of error probability in the low and zero predictability sentences. The contribution of semantics was computed as the ratio of the logarithms of error probability in high and low predictability sentences. The resulting parameterkis the amount by which the number of independent channels of acoustical information would need to be increased in order to produce the measured increase of recognition probability. Average values ofkwere 1.4 and 2.0 for syntax and semantics, respectively. Statistical analysis revealed thatkwas independent of S/N ratio. [Work supported by PSC. CUNY award 6‐63137.]