The intelligibility of speech uttered in a hyperbaric helium-oxygen (heliox) respiratory mixture is affected in the main by an overall nonlinear frequency translation of the speech spectrum. Spectral resonances (formants) in the short-time spectrum of voiced speech in particular have generally been characterised by a deterministic nonlinear shift curve relating form ant centre frequencies in normal air speech to their corresponding frequency locations in helium speech. New results are presented which, whilst supporting the general principle of a nonlinear formant shift in heliox, are the antithesis of the classical theory relating to the formant shift characteristic. It is shown that different speech sounds (phonemes) exhibit characteristic formant shift profiles independent of gas mixture and pressure. These results imply that the helium speech characteristic is affected not only by the properties of the respiratory environment itself, but also by apparently deliberate attempts by the diver to render his own speech intelligible to himself as he perceives it.