This study contributes to innovation and diffusion models by examining phonetic changes in London English. It evaluates Sapir's notion of “drift,” which involves “natural,” unconscious change, in relation to these changes. Investigating parallel developments in two related varieties of English enables drift to be tested in terms of the effect of extralinguistic factors. The diphthongs of price, mouth, face, and goat in both London and New Zealand English are characterized by “Diphthong Shift,” a process that continued unabated in New Zealand. A new, large data set of London speech shows Diphthong Shift reversal, providing counterevidence for drift. We discuss Diphthong Shift and its “reversal” in relation to innovation, diffusion, leveling, and supralocalization, arguing that sociolinguistic factors and dialect contact override natural Diphthong Shift. Studying dialect change in a metropolis, with its large and linguistically innovative minority ethnic population, is of the utmost importance in understanding the dynamics of change.