By means of time-resolved small-angle X-ray scattering, we developed an analysis methodology to assess the void volume fraction ϕv in high-density polyethylene (HDPE) during tensile testing. The specimens were first drawn up to different imposed strains, and subsequently were subjected to stress unloading and strain recovery stages. During the loading stage, ϕv progressively increased with the strain level, starting from a well-defined onset strain prior to the yield point. In particular, ϕv reached a maximum of 8.75 vol% for a strain of 12.5% in the case of a HDPE grade with a molecular weight of 105 000 g mol−1. Stress unloading and strain recovery caused a decrease in ϕv attained at the end of the loading stage. For a HDPE grade with a molecular weight of 55 000 g mol−1, ϕv was more important during the loading stage and the decrease in ϕv was less marked during the stress unloading stage when compared to the HDPE with molecular weight of 105 000 g mol−1. The residual and reversible components of void volume fraction were revealed.