The 129Xe nuclear spin polarization (PXe) that can be achieved via spin-exchange optical pumping (SEOP) is typically limited at high in-cell xenon densities ([Xe]cell), due primarily to corresponding reductions in the alkali metal electron spin polarization (e.g. PRb) caused by increased non-spin-conserving Rb–Xe collisions. While demonstrating the utility of volume holographic grating (VHG)-narrowed lasers for Rb/129Xe SEOP, we recently reported [P. Nikolaou et al., JMR 197 (2009) 249] an anomalous dependence of the observed PXe on the in-cell xenon partial pressure (pXe), wherein PXe values were abnormally low at decreased pXe, peaked at moderate pXe (~300 torr), and remained surprisingly elevated at relatively high pXe values (>1000 torr). Using in situ low-field 129Xe NMR, it is shown that the above effects result from an unexpected, inverse relationship between the xenon partial pressure and the optimal cell temperature (Topt) for Rb/129Xe SEOP. This interdependence appears to result directly from changes in the efficiency of one or more components of the Rb/129Xe SEOP process, and can be exploited to achieve improved PXe with relatively high xenon densities measured at high field (including averaged PXe values of ~52%, ~31%, ~22%, and ~11% at 50, 300, 500, and 2000 torr, respectively).