This paper examined the intelligibility of English /s/ and /esh/ followed by the vowel /ilengthmark/ produced by 20 native speakers of Japanese (JE /s/, JE /esh/), and the acoustic characteristics of JE /s/ and /esh/ according to the different levels of intelligibility. Five native English speakers evaluated the intelligibility of JE /s/ and /esh/ in the word-initial position of "seat" and "sheet." The major energy peak locations were analyzed for /s/ and /esh/ produced by Japanese speakers and native English speakers, and for the Japanese sibilant /ctc/.

It was found that the overall intelligibility was lower for JE /esh/ than for /s/ and that JE /s/ and /esh/ with low intelligibility were heard as /esh/ and /s/, respectively. When the intelligibility was lower, JE /s/ had energy peaks in the lower frequencies, and JE /esh/, in the higher frequencies, having characteristics similar to /ctc/. L1 transfer occurred in the pronunciation of English /s/ and /esh/ by Japanese speakers.