PURPOSE: An electrical desalting auxiliary for crude petroleum exhibiting good water recovery and desalting when added in a small amt., which comprises a specified polyoxyalkylenealkylphenol-formaldehyde condensate contg. a polyoxyethylene chain in the molecule.

CONSTITUTION: An electrical desalting auxiliary comprising a polyoxyalkylenealkylphenol-formaldehyde condensate of average MW of 1,400W2,800 calculated excluding the polyoxyalkylene chain, contg. 45W70wt% polyoxyethylene chain in the molecule and having an 8W12C alkyl group. The polyoxyalkylenealkylphenol-formaldehyde condensate to be used has formula I (wherein R is 8W12C alkyl; R1 is 3W5C alkylene; each of m1, m2 and m3 ≥0; each of x, n1, n2 and n3 ≥1). It is important that the alkylphenol-formaldehyde condensate which is removed from the polyoxyalkylene chain in the chemical structure of the formula I, namely the portion of formula II has an MW of 1,400W2,800, otherwise the performance remarkably deteriorates.