A technique for constructing optimally smooth model rotation curves has been applied to complexes of HII regions and molecular clouds, yielding a Galactocentric distance of R₀ = 8.3 ± 0.8 kpc. The residual motion of the Local Standard of Rest with respect to these objects is 3.4 ± 1.3 km s^-1 in the direction of the Galactic anticenter and 4.1 ± 2.1 km s^-1 in the direction of the Galactic rotation. Ignoring the azimuthal component of this motion results in substantial underestimation of R₀. Data on the rotation of neutral-hydrogen and HII-region subsystems and the derived value of R₀ are used to estimate the Oort constant A = 16.2 ± 0.6 km s^-1 kpc^-1 and the parameter AR₀ = 135 ± 5 km s^-1. Analysis of a combined sample of molecular clouds associated with HII regions or reflection nebulae reveals a clear difference between the R₀ values inferred from data for northern (l < 180°) and southern (l > 180°) objects, similar to the known difference for OB stars.