[en] The rate of depolarisation of the linear polarisation of the average pulse profil is a function of pulsar period. Short period pulsar depolarise less rapidly and show a variation of depolarisation rate across their pulse profiles. The trailing edge depolarises more rapidly than the leading edge. This is paralleled by a front to back asymetry of the total intensity profile. These latter effects are presumably due to distortion of the magnetosphere near the velocity of high cylinder. The predominant depolarisation mechanism is apparently different for the long and short period pulsars. Randomisation of the position angle of polarisation during propagation in the weaker fields of the outer magnetosphere may account for the depolarisation of the long period pulsars. An increased frequency of occurence of orthogonally polarised radiation may be responsible for the depolarisation of the short period pulsars. The majority of pulsar (> 70% in this sample) show depolarisation of their circular polarization at high frequency. There are indications that this is period dependent. (orig.)