A system for crude oil desalting and dehydration in a single vessel is disclosed. The system comprises a pressure vessel defining a cavity and comprising a sealed bulkhead separating the cavity into a first compartment and a second compartment. Disposed within the first compartment is a first distributor disposed within the first compartment, a first electrical grid assembly, and a transfer conduit that passes through the bulkhead from the first compartment to the second compartment, the second compartment being in fluid communication with the first compartment via the transfer conduit. The pressure vessel may further comprise an overflow weir in the first compartment and a second electrical grid assembly disposed within the second compartment and aligned substantially transverse to a flow path extending from the transfer conduit to the crude collection header.