In this paper we investigate the behaviour of the lexical rising tone (SAC) in disyllabic sequences in the Northern variety of Vietnamese. Results from task-oriented dialogues show that this rising tone (SAC), when occurring before the lexical high-level tone (NGANG), can be realised as low level or falling, resembling a different tone in the language (HUYEN). This is the case word-internally and within noun phrases. Two further observations give us an indication that a sandhi process could be developing: (a) this variation is not found in sequences across a larger juncture, and (b) the SAC tone does not undergo this change before other tones.