This dissertation investigates the hypothesis that the more vowel-like a consonant is, the more difficult it is for listeners to classify it as geminate or singleton. A perceptual account of this observation holds that more vowel-like consonants lack clear markers to signal the beginning and ending of the consonant, so listeners don’t perceive the precise duration and consequently the phonological contrast may be neutralized in some languages. Three experiments were performed to address these questions using data from Persian speakers. In Experiment I, four speakers produced singleton and geminate tokens of the voiced oral consonants [d,z,n,l,j] and the glottals [h] and glottal stop at three speaking rates. It was found that Persian speakers do distinguish geminate durations from singleton durations for all manners even at very fast speaking rates, and vowels preceding geminates are slightly longer than those preceding singletons. Speaking rate had more of an effect on geminates than on singletons for all segments studied: the durations of the geminates decreased more in fast speech than the durations of the singletons did. In Experiment II, listeners heard manipulated continua of consonants ranging from singletons to geminates. Subjects’ identification curves were modeled using the cumulative Gaussian model. The modeled standard deviation was interpreted as the breadth of the perceptual threshold, and a broader threshold understood to indicate a less distinct perceptual boundary between the two categories. Obstruents [d,z] had smaller breadth values than the sonorants [n,l,j], and the glottals had the largest breadth values of all. This indicates that while sonorants were more difficult for listeners to categorize than obstruents, the glottals were the most difficult to categorize of the segments tested. Experiment III tested whether the modification of a specific parameter, the formant transition duration, would affect the perceptibility of the geminate/singleton contrast. A single token containing the glide [j] was manipulated to produce three different continua, each having a distinctly different manipulated transition: short, normal or long. It was found that the longer the transition was, the broader the perceptual threshold, thus making the consonant harder to categorize. / text