GO:0070940

The process of removing a phosphate group from an amino acid residue in the C-terminal domain of RNA polymerase II. Some dephosphorylation occurs during transcription while some may occur after the enzyme is released from the template in order to prepare it for the beginning of the transcription cycle again. RNA polymerase II with little or no phosphorylation is referred to as the hypophosphorylated or II(A) form. [GOC:krc, GOC:mah, PMID:17079683]