EXHIBIT 21 MARKWEST HYDROCARBON, INC. LIST OF SUBSIDIARIES NAME OF SUBSIDIARY TYPE OF ENTITY RELATION - ------------------ -------------- -------- MarkWest Michigan, Inc. Colorado Corporation Wholly-owned subsidiary West Shore Processing Company, LLC Michigan Limited Liability Company Wholly-owned subsidiary Basin Pipeline, LLC Michigan Limited Liability Company Wholly-owned subsidiary MarkWest Resources, Inc. Colorado Corporation Wholly-owned subsidiary Matrex, LLC Michigan Limited Liability Company Wholly-owned subsidiary