To perform effectively, electrostatic precipitators require a highly uniform velocity distribution in the treatment zone. This requirement places particular demands on the design of the wide-angle diffuser upstream of the treatment zone. To generate satisfactory diffuser exit velocity profiles, perforated plates are installed within the diffuser. This paper provides a report of an experimental investigation of the nature of the flow generated by installing two perforated plates in the diffuser. Detailed measurements of the static pressure distribution and flow patterns are reported. The effects of altering the porosity and locations of the perforated plates on the flow and pressure loss characteristics are considered.