The evolution of argillaceous rock suffering from desaturation/resaturation is a key factor that affects the safety of deep geological repositories of radioactive waste. However, the behavior of this rock when resaturated to nearly 100% relative humidity (RH) has been poorly studied, because water condensation was frequently encountered in the conventional RH-control technique. This issue has been overcome in the present work using an environmental scanning electron microscope (ESEM) that provides improved RH control. The recorded micrographs subsequently were analyzed for full-field strain measurement using digital-image correlation techniques. The results reveal that the swelling (the sample in the size of several millimeters) attained stability in less than an hour during the 30%���80% RH step; however, the swelling strain continued to rise for up to 10h during the 80%���99% RH step. The anomalous time-dependent swelling at nearly 100% RH is not explicable in terms of hydraulic flow or propagation of swelling-induced microcracks. The evolution of strain fields shows that the areas of continuous swelling are located mainly inside the clay matrix. We suggest that the creep-like swelling of clay might be related to an osmotic flow (driven by a solute-concentration gradient) that dissipates more slowly than the hydraulic flow (driven by pressure gradient). The existence of time-dependent swelling in argillaceous rock under resaturated conditions should be considered for the reliability assessment of long-term radioactive waste disposal.