Dilute neutral polymer solutions provide an effective environment for the electrophoretic separation of polyelectrolytes by molecular weight. To better understand the mechanisms responsible, this study examines how the molecular weight dependent mobility mu of poly(styrenesulfonate), termed the probe chain, depends on the molecular weight and concentration of various dilute pullulans, termed the host chains. Available for this system are nearly monodisperse probes and hosts. Carefully performed capillary electrophoresis experiments measure absolute values of mu and its distribution. Below the overlap concentration, hosts reduce mu from its free solution value in proportion to concentration. The strongest dependence of mu on probe molecular weight occurs at nearly equal probe and host chain lengths. The influence of host and probe chain lengths on mu can be understood in terms of pairwise probe-host configurational interactions. Three parameters-entanglement probability, average entanglement duration, and average entanglement displacement-capture the interaction dynamics and help to explain observed variations in mu.