Abstract We present simultaneous measurements of emission from dust continuum at 230 GHz and the J = 2–1 12 CO, 13 CO, and C 18 O isotopologues at ∼15 pc resolution from individual giant molecular clouds (GMCs) in the Andromeda galaxy (M31). These observations were obtained in an ongoing survey of this galaxy being conducted with the Submillimeter Array. Initial results describing the continuum and 12 CO emission have been published earlier. Here, we primarily analyze the observations of 13 CO and C 18 O emission and compare them to the measurements of dust continuum and 12 CO emission. We also report additional dust continuum and CO measurements from newly added GMCs to the M31 sample. We detect spatially resolved 13 CO emission with high signal-to-noise ratios in 31 objects. We find the extent of the 13 CO emission to be nearly comparable to that of 12 CO, typically covering 75% of the area of the 12 CO emission. We derive 13 CO and C 18 O abundances of 2.9 × 10 −6 and 4.4 × 10 −7 relative to H 2 , respectively, by comparison with hydrogen column densities of the same regions derived from the dust continuum observations assuming a Milky Way gas-to-dust ratio. We find the isotopic abundance ratio 13 CO/C 18 O = 6.7 ± 2.9 to be consistent with the Milky Way value (8.1). Finally, we derive the mass-to-light conversion factors for all three CO species to be α 12 = 8.7 ± 3.9, α 13 = 48.9 ± 20.4, and M ⊙ (K km s −1 pc 2 ) −1 for the J = 2–1 transitions of 12 CO, 13 CO, and C 18 O, respectively.