Thin‐film growth and compositional effects ofc‐axis oriented YBa2Cu3O7−x(YBCO) thin films synthesized by metalorganic chemical vapor deposition have been investigated. The formation of single cation films using tetramethylheptanedionate precursors was shown to be mass controlled, exhibiting a ratio of deposited to evaporated species in the increasing order Ba, Y, and Cu. The physical properties of off‐stoichiometric YBCO films deposited on MgO substrates were measured in the compositional range 1.1≤Ba/Y≤2.3 and 1.5≤Cu/Ba≤4.6. While structural properties such asc‐axis values and rocking curves appeared unaffected to variations in cation stoichiometry, morphology was observed to be extremely sensitive even to slight changes in composition. Off‐stoichiometric layers with Cu/Ba≳1.5 were observed to exhibit Cu‐rich precipitates embedded in a 1:2:3 YBCO film matrix. The zero‐resistivity temperatures were above 77 K for all cation film compositions measured. However, sharp ac‐susceptibility transitions were restricted to a more narrow compositional range (1.9<Cu/Ba<3.6). The best superconducting properties [Tc=85 K,Jc(77 K)≳106A/cm2] were observed for films with relatively rough surface morphologies (Ba/Y=1.6 and Cu/Ba=3.5). An optimum trade‐off between smooth surfaces and superconducting properties was found for Ba/Y=1.5 and Cu/Ba=1.9, yieldingTc=81 K andJc(77 K)=3×105A/cm2.