Scottish Gaelic, an endangered Celtic language, demonstrates alternations in word-initial consonants, as in “pòg” [phok] 'kisses' vs “phòg” [fok] 'kissed.' This process, called lenition, leads to apparent neutralizations of Gaelic segments, for example of the [f] of “ phòg” with [f] of “foghlam” [foɫəm] “education,” which is not caused by lenition. A perception experiment can show whether listeners hear any residual difference between lenited segments (e.g., [f]<-[ph]) and the phonetically similar segments ([f]<-[f]). This project used a gating study to investigate when in the word listeners determine which type of sound they are hearing. Preliminary results from 17 native Gaelic listeners indicate that listeners cannot distinguish lenited from phonetically matched consonants (e.g., the two types of [f]) from cues in the consonant itself, but can distinguish both from the unlenited phonologically matched consonants (e.g., [ph]) very accurately. Listeners become able to distinguish lenited from phonetically matched segments (the two types of [f]) during either the following vowel or the segment after that, depending on what coarticulatory cues with the latter parts of the word are available. Thus, listeners need enough acoustic information to provide lexical disambiguation in order to determine the morphological source of lenited sounds.