This thesis presents a socio-phonological analysis of three diphthongs in Mersea Island English (MIE). Mersea Island is situated off the North East coast of Essex in South East England. Socio-economically, the Island has seen dramatic change over the past century. Originally quite isolated and economically largely self-sufficient, social and demographic changes have led to significantly greater contact (both social and linguistic) with the mainland. Thus, parallels may be drawn between the developments of Mersea Island and those of, for example, Martha's Vineyard and Ocracoke Island in North America. The nature of these social changes will be evaluated alongside the analysis of the diphthongs MOUTH, PRICE and CHOICE. As a result, the direction of change and typologies of variation present in the speech of three generations of Mersea Islanders will be established, and the relationship between the dialectological findings and external sociolinguistic factors will be explored. The results from the sociolinguistic data analysis will then be considered in light of naturalness and phonological theory. Using an approach combining the mechanics of Optimality Theory (OT) and the principles of Dispersion Theory, a three-tiered model will be constructed which effectively represents the interface between internal (linguistic) factors and external (sociolinguistic) influences on language variation and change. It will be seen, through the application of this model to MIE that, while some variation may be generated at the linguistic level, other variation may be attributed to selections based upon sociolinguistic considerations. In addition, the incorporation of variation at each level of the model allows for a distinction to be made between language change, which is generated through linguistic motivations, and change which is motivated by external sociolinguistic desires.