This thesis seeks to analyse two types of linguistic features of Welsh, code-switching and mutation, as sociolinguistic variables: features which encode social information about the speaker and/or stylistic meaning. Developing a study design that incorporates an analysis of code-switching and mutation in naturalistic speech has demanded a relatively novel methodological approach. The study combined a variationist analysis of the vernacular use of both variables in the 40-hour Siarad corpus (Deuchar 2014) with a technique that ranks radio programmes in order of formality through the use of channel cues and other criteria (Ball et al 1988). This allows for a comparison of the use of code-switching and mutation in multiple stylistic contexts, each of which show varying degrees of emotional engagement and self-monitoring by speakers. The analysis found that code-switching was strongly correlated with the level of formality of each radio programme, and that at least one aspirate mutation trigger, (a), also patterned in a similar way. Some other mutation triggers, most notably including the nasal possessive trigger (fy), seemed to be primarily affected by the speakers’ backgrounds and their relative ages in particular. A qualitative analysis of the type of discourse found in each radio programme made further links between the institutional style of each programme and their use of the stylistically controlled ‘marker’ variables, with non-standard variants appearing to be indexical of solidarity, subversion and irony, while standard variants indexed prestige, authority and earnestness.