The transient response of electrorheological suspensions in shear flow subjected to a suddenly imposed electric field is investigated experimentally. Barium titanate/silicone oil and alumina/mineral oil suspensions are employed. The evolution of both the rheological properties and the suspension structure are investigated. Results are compared with predictions from a two-fluid continuum model reported previously. Transient responses appear above a critical field strength, and the critical Mason number for the onset of a transient rheological response is equivalent to the critical Mason number for the onset of lamella formation, within experimental uncertainty. These results are consistent with predictions. The experimentally determined values of the critical Mason number agree with those predicted, with differences of the order of the experimental uncertainty. However, we find that the critical Mason number depends on shear rate, rather than being independent of shear rate as predicted. (C) 2008 The Society of Rheology.