Studies of duration in Welsh have concentrated on specific environments, such as word-final, pre-final and pre-pre-final positions, in relation to the positions of linguistic stress and phonetic pitch accent. The present approach is broader, in that it looks at the temporal patterning of syllable and foot sequences. First, general global and local measures of isochrony and irregularity are applied (standard deviation; pairwise variability). Second, since isochrony is a necessary but not a sufficient correlate of rhythm, a new algorithm is introduced for characterising grouping properties. Third, syllable sequences are segmented into feet using alternative duration relations and examined for their relation to grammatical groups. It appears that Welsh duration patterns are neither syllable-timed nor foot-timed in the accepted sense, but have what may be called a “rallentando timing”, in which sequences of increasing length mark grammatically relevant prosodic units.