This disclosure provides a method of determining a pore throat size distribution of reservoir rock, comprising injecting a set of different-sized probe particles through a portion of reservoir rock and measuring retention volumes or times of each of the different-sized probe particles eluting from the portion of reservoir rock. This disclosure also provides a testing apparatus comprising a probe tube for receiving a set of different-sized probe particles in an elution fluid returning from a portion of reservoir rock and an analytical module connected to receive the elution fluid from the probe tube. The module includes a flow cell to receive the elution fluid from the probe tube, a radiation source positioned to direct a radiation beam though a window of the cell into the eluting fluid in the cell, and a sensor for detecting portions of the radiation beam that have interacted with the different-sized probe particles such that retention volumes or times each of the probe particles of one size class can be distinguished from all other size classes of the set.