This study examines the (morpho-)phonological variation of Jakarta Indonesian (JI) spoken in the capital of Indonesia. On the basis of ancillary information, we assume that JI developed from contact between Standard Indonesian (SI) and Betawi with influence of Javanese. The patterns of variation found in naturalistic speech corpora from three generations of speakers of JI (Wallace, 1976; Gil et al., 2015) indicate that JI is emerging as a new variety of Indonesian. These corpora give evidence of the changes that are taking place, their direction, and how they are adapted by both genders, and the various age and social groups represented in the corpus. These facts have implications for understanding the social structure of the community. There are three variables under investigation. Chapter Two examines variants with final [-a] ~ [-e] in function words, such as in [apa] (SI) ~ [ape] (Betawi) ‘what’, Chapter Three investigates variants with final [Ø] ~ [-h] ~ [-ʔ] in function words, such as in [lagi] (SI) ~ [lagih] (Betawi) ~ [lagiʔ] (Betawi) ‘more/progressive’, in content words, such as in [sapi] (SI) ~ [sapiʔ] (Betawi) ‘cow.’ Chapter Four studies the patterns of variation of the active verbal prefix focusing on the variation with voiced obstruent initial roots including [ŋə-] and variants with nasal assimilation, as in [ŋə-bəli] (associated with Betawi) ~ [m-bəli] (associated with Javanese) ‘to buy.’ The high occurrences of word-final [-a] in Chapter Two and word-final [Ø] in Chapter Three show evidence of strong influence of SI. The high occurrences of the variants with nasal assimilation in active verbal prefix show evidence of Javanese influence. The observed patterns of variation are primarily conditioned by social factors, namely speakers’ gender and level of education. The increased use of SI forms in Chapters Two and Three and Javanese form in Chapter Four are led by females and speakers of higher educational background. The increased use of these forms can be seen as a change in progress influenced by the varieties that have more prestige: (1) SI, as the standard variety; (2) Javanese, which is associated with a group with prestige in Jakarta.