This dissertation explores the phonetic mechanisms of the sound change known as rhotacism (/s/ greater than /z/ greater than /r/) which is observed in Italic, Germanic, and Sanskrit, among other languages, employing lab-based methods of "experimental historical phonology" (Ohala 1974), and approaching sound change from the theoretical standpoint of Evolutionary Phonology (Blevins 2004). Rhotacism is viewed as a phonetically motivated regular sound change which has its source in the listener's reanalysis of phonologically ambiguous segments (Ohala 1993). Assuming that reflexes of PIE */s/ and */r/ in Latin and Germanic were realized non-canonically due to coarticulation effects at the historical moment when the sound change occurred, I propose that rhotacism is the result of the listener reanalyzing phonetic realizations of a target /z/ in the speech signal as belonging to /r/. I conduct two experiments which investigate the acoustic and auditory properties of intervocalic voiced sibilant fricatives and voiced rhotics in English and Polish, examining the effects of coarticulation by manipulating the vocalic environment and speech rate. Results of a production experiment demonstrate that there is natural similarity between postalveolar fricatives and rhotics in terms of acoustic properties such as duration, intensity, formant transitions, etc. The results of a listening experiment show that listeners may hear some voiced fricatives as more auditorily similar to certain rhotic tokens than others. While neither experiment directly addresses the issue of phonological reanalysis in rhotacism, I conclude that the phonetic properties of the segments involved are initially similar to such an extent that further lenition may engender a situation where listeners could analyze specific realizations of /z/ for a speaker-intended /r/. The six chapters of the work cover the following: the range of sound changes described as rhotacism; phonetic motivations for the change; an Ohalian listener-based account; rhotacism in Italic (Latin, Faliscan, Umbrian); rhotacism in West and North Germanic, with a focus on the phonetics of */r/ reconstructed in Germanic; an acoustic study of voiced sibilant fricatives and rhotics; a pilot study exploring listener perception of similarity between fricatives and rhotics; a conclusion and discussion of further research. [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.]