In sociolinguistics, language variation in vowel sounds is typically studied using phonetic transcription. Phonetic transcription is carried out by expert listeners, who are capable of perceptually separating (socio-) linguistic variation from anatomical/physiological speaker-related characteristics. However, phonetic transcription is a very laborious task and its reliability is questionable. In phonetics, vowel normalization procedures were designed to separate linguistic variation from anatomical/physiological variation in acoustic measurements. In this thesis, it is evaluated whether vowel normalization procedures are suitable for use in sociolinguistics, for studying language variation in vowels. It is described how 12 procedures for vowel normalization were compared. This comparison consists of two parts. First, the procedures were evaluated in the acoustic domain by applying them to acoustic (formant and fundamental frequency) measurements of a database of vowels pronounced by 160 Dutch teachers from the Netherlands and Flanders. The procedures were evaluated on how well they preserved (socio-) linguistic variation and minimized anatomical/physiological variation. Second, the normalization procedures were compared in a perceptual-acoustic comparison; a comparison with perceptual judgments made by expert listeners. The normalization procedures were applied to the acoustic measurements of vowel tokens from 20 of the 160 Dutch teachers. These tokens were also judged by expert listeners on their perceived (socio-) linguistic characteristics, i.e., tongue height, tongue position, and lip rounding in a listening experiment. Next, the transformed acoustic measurements were compared to the perceptual judgments using linear regression analysis, to evaluate how well the output of the normalization procedures modeled the perceived (socio-) linguistic characteristics of the vowel tokens. These two comparisons of the 12 normalization procedures show that procedures that incorparate information (e.g., the mean or ...