Epitaxial thin films of Pb(Mg1/3Nb2/3)O3 (PMN) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) were deposited on (0 0 1) SrTiO3 and SrRuO3/SrTiO3 single crystal substrates at 700°C by the solid-source MOCVD method, using tetramethylheptanedionate (THD) sources of Pb, Mg, Nb, and Ti. Sputtered SrRuO3 was employed as an epitaxial bottom electrode layer so that electrical property measurements could be made. Pure PMN films were prepared using pre-mixed source material (5.5 moles of Pb(THD)4,1 mole of Mg(THD)2 and 2 moles of Nb(THD)4). XRD results indicated that these films were single phase and had the pervoskite structure. Rutherford Backscattering showed that Pb/Mg/Nb/O=18%/7%/13%/62%. Films of PMN-10%PT and PMN-20%PT films were also epitaxially deposited on the same substrates. The dielectric constants for these films were in the range 1200-1500 and 600-700, respectively. A 150 nm thick PMN-20% PT film was strongly dispersive. The dielectric constant for a 150 nm thick film decreased from 700 at 1 kHz to 550 at 100 kHz. The dielectric loss factor increased from 0.15 to 0.4 with increasing frequency.