We reported previously that onset of oligodendrocyte precursor cell (OPC) differentiation is accompanied by an increase in intracellular pH (pH_i). We show that OPC differentiation is dependent primarily on a permissive pH_i value. The highest differentiation levels were observed for pH_i values around 7.15 and inhibition of differentiation was observed at slightly more acidic or alkaline values. Clamping the pH_i of OPCs at 7.15 caused a transient activation of ERK1/2 that was not observed at more acidic or alkaline values. Furthermore, inhibition of ERK activation with the UO126 compound totally prevented OPC differentiation in response to pH_i shift. These results indicate that pH_i, acting through the ERK1/2 pathway, is a key determinant for oligodendrocyte differentiation. We also show that this pH_i pathway is involved in the process of retinoic acid‐induced OPC differentiation. © 2006 Wiley‐Liss, Inc.