In this paper we investigate category-speciﬁc effects throughthe lens of Welsh mutation. Smith (2011) and Moreton et al. (2017) show that English distinguishes nouns and proper nouns in an experimental blending task. Here we show that Welsh distinguishes nouns, verbs, personal names, and place names in the mutation system. We demonstrate these effects experimentally in a translation task designed to elicit mutation intuitions and in several corpus studies. In addition, we show that these effects correlate with lexical frequency. Deeper statistical analysis and a review of the English data suggests that frequency is a more explanatory factor than part of speech in both languages. We therefore argue that these category-speciﬁc effects can be reduced to lexical frequency effects.