Metathesis has often been described as abrupt and sporadic and thus as an exception to neogrammarian sound change (e.g. Wang 1969), while others suggested metatheses to be gradual, phonetically based developments (e.g. Blevins & Garrett 1998). Andalusian Spanish provides an opportunity to study this issue. As in many other Spanish varieties, in Andalusian Spanish, syllable final /s/ is usually weakened to [h] or even deleted (pasta ‘paste/pasta’ [ˈpahta]). Traditional dialectological or sociolinguistic studies used to transcribe medial /sp, st, sk/ as pre-aspirated stops [hp, ht, hk] or geminates [pp, tt, kk]. However, post-aspirated stops [ph, th, kh] have recently been reported for Western Andalusian Spanish (Torreira 2007a, 2012; Parrell 2012). Ruch & Harrington (submitted) found that younger Andalusian speakers produced /st/ with an important amount of post-aspiration, while older speakers produced a longer pre-aspiration. In this study 11 Spanish words with medial /st/ followed by different vowels (e.g. pestiño, pestaña, estufa) are analysed in order to investigate whether the emergence of post-aspiration is favored by phonetic factors and/or by lexical frequency. By a semi-automatic procedure voice offset time (VOffT) was measured for pre-aspiration, and voice onset time for post-aspiration in the materials produced by 48 speakers (24 from Seville, Western Andalusia, and 24 from Granada, Eastern Andalusia, both divided into two age groups). In all words analyzed, younger speakers produced a shorter VOffT and a longer VOT than older speakers. The analysis of linguistic factors showed that a following high front vowel /i/ favored a longer VOT in all four speaker groups. /st/-sequences followed by /u/ showed a longer VOT in older, but a shorter VOT in younger speakers, compared to the other two phonological contexts _a and _i. Further analysis indicated an interaction between lexical frequency and phonological context, the influence of the latter being more marked in less frequent words. Our results suggest that metathesis can be the result of a regular and gradual process, which is, in the beginning, favored by phonetic factors. Only as the sound change advances further, lexical factors gain in importance.