This dissertation presents an instrumental study of variation in fricative voicing in Buenos Aires Spanish (BAS), particularly with respect to the devoicing change of the postalveolar fricative: /y/greater than/[function of]/. It proposes a novel way of determining the completion of this change by comparing the percentage voicing of the postalveolar fricative to that of /s/, thus providing a system-internal benchmark for voicelessness in a language without a fricative voicing contrast. The findings from the production study show that there is still much variation in the voicing of /y/ that is both socially and phonetically conditioned. Although the change has reached completion only in the younger middle class group, all other social groups show evidence that the devoicing change is still progressing. Cross language discrimination experiments using Portuguese stimuli were conducted to explore the effects that socially conditioned variation and positionally defined allophonic variation in obstruent voicing may have on the delimitation of perceptual categories used to discriminate non-native obstruent voicing contrasts. The results from the ABX task show that, instead of improving the discrimination of Portuguese /[function of]/-/y/, having socially differentiated variation in the native speech community actually hinders the discrimination of this contrast, regardless of the listener's social background. Listeners, regardless of age and social class, do not seem to tap into the socially conditioned differential distribution of /[function of]/ and [y] in BAS in order to discriminate the non-native contrast. The goodness rating task results show that listeners who maintain the voicing of /y/ in their production give Portuguese /[function of]/ significantly lower ratings than Portuguese /y/. However, the converse is not true: listeners for whom the /y/greater than/[function of]/ change has been completed give similarly high ratings to both /y/ and /[function of]/. Rather than being determined by social differences, the evaluation of the variation (/[function of] [approximately] y/) seems to be affected by the listener's own production. The more innovative speakers in the devoicing change seem to have a more expansive postalveolar fricative perceptual category, whereas the conservative speakers' category has a smaller range of acceptable realizations. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]