The differential solubility of man-made cellulosic fibres in zinc chloride/formic acid dissolutions has been studied. This technique of characterisation of the fine structure has been applied to substrates of different families of cellulosic man-made fibres such as viscose (staple and filament), modal, polynosic and lyocell fibres. The developed technique has allowed the detection of the fine structure differences in substrates of different families of man-made cellulosic fibres, and even in substrates of the same family and/or form of presentation (staple fibre, filament yarn).