Phase behavior of shale fluids in small pores is not well understood. One complexity comes from the fact that sorption of components by organic-rich shale can be significant and selective. In an attempt to elucidate the effect of sorption on phase behavior of shale fluids, we present a new experimental method that can be used to measure the bubble-point pressures of N2/n-C4H10 mixtures in the presence of an actual shale sample. Pressure/volume (P/V) isotherms for a given mixture were firstly measured in a PVT cell. Then, the measurements of the P/V isotherms for the same mixture were repeated in a partially confined space by opening a valve between the PVT cell and a shale container. The so-called partially confined space consists of the pore space inside the shale sample, the bulk space in the PVT cell and in the connecting tubing, and the non-cementing pore spaces among the shale particles. Results show that, the measured bubble-point pressure of the N2/n-C4H10 mixture in the partially confined space was higher than the corresponding bubble-point pressure in the bulk space. A detailed analysis indicates that, when loaded in the partially confined space, n-C4H10 exhibits a higher level of sorption capacity on the shale sample than N2, resulting in a higher concentration of N2 left in the free fluid than that in the initial mixture, which is the so-called selective sorption. The higher concentration of N2 led to the higher bubble-point pressure as observed in the measurements. The increase of the bubble-point pressure due to the selective sorption was observed to be greater at a lower temperature. This is because the sorption of n-C4H10 relative to that of N2 is more significant at a lower temperature. A higher temperature did not lead to a higher increment in the bubble-point pressure likely because bubble-point is more sensitive to composition than to temperature for these mixtures at the conditions tested. This emphasizes the importance of considering sorption in phase behavior for small pores.