‘Phonologisation’ is a process whereby a phonetic phenomenon enters the phonological grammar and becomes conceptualised as the result of categorical manipulation of phonological symbols. I analyse the phonologisation of a predictable phonological pattern in Welsh, with particular attention to identifying criteria for whether phonologisation has occurred. I argue for a model where phonologisation experiences bottom-up and top-down biases. From the bottom up, there is pressure to phonologise phenomena with a categorical distribution; from the top down, there exist formal constraints on featural specification. I focus on the requirement for featural specifications to obey the Contrastivist Hypothesis, which denies that redundant features can be involved in phonological computation, in the context of a framework with emergent features. I suggest that the Contrastivist Hypothesis provides a useful boundary condition for emergent-feature theories, whilst independent phonologisation criteria provide contrastivist approaches with a more solid conceptual underpinning.