The separation of membrane proteins by high-resolution two-dimensional electrophoresis was carried out. At high loads, these proteins are prone to precipitation, resulting in poor resolution. It is shown here that the use of thiourea, previously described for focusing in immobilized pH gradients, can be extended to conventional isoelectric focusing. As thiourea inhibits acrylamide polymerization, a modified photopolymerization system must be used. These modifications result in higher solubility of proteins during IEF, thereby increasing the resolution and capacity of the two-dimensional gels.