AbstractThree grades of polyethylene, with weight-average relative molar masses, ¯¯¯¯MW, of approximately 0.6 × 106, 5 × 106, and 9 × 106, were supplied as compression mouldings by a leading manufacturer of ultra-high molecular weight polyethylene (UHMWPE). They were code-named PE06, PE5, and PE9, respectively. Specimens cut from these mouldings were subjected to a wide range of mechanical tests at 23 °C. In tensile tests, deformation was initially elastic and dominated by crystallinity, which was highest in PE06. Beyond the yield point, entanglement density became the dominant factor, and at 40 % strain, the rising stress–strain curves for PE5 and PE9 crossed the falling PE06 curve. Fracture occurred at strains above 150 %. Differences in stress–strain behaviour between PE5 and PE9 were relatively small. A similar pattern of behaviour was observed in wear tests; wear resistance showed a marked increase when ¯¯¯¯MW was raised from 0.6 × 106 to 5 × 106, but there was no further increase when it was raised to 9 × 106. It is concluded that the unexpected similarity in behaviour between PE5 and PE9 was due to incomplete consolidation during moulding, which led to deficiencies in entanglement at grain boundaries; they were clearly visible on the surfaces of both tensile and wear specimens. Fatigue crack growth in 10 mm thick specimens was so severely affected by inadequate consolidation that it forms the basis for a separate report – Part 4 in this series.