CBS-Q//B3, G4MP2, and G4 composite method calculations were used to estimate gas phase standard state (298.15 K, 1 atm) free energies of hydration (Δ~hydr~G°~(g)~), hydration equilibrium constants (log K~hydr,(g)~), bond dissociation enthalpies (BDEs), and enthalpies (Δ~d~H°~(g)~) and free energies (Δ~d~G°~(g)~) of aldehydic proton acid dissociation for various substituted aldehydes with electron withdrawing and electron releasing groups. Good quality log K~hydr,(g)~ correlations with the Swain-Lupton resonance effect parameters R and R^+^ were found, allowing extension of the model to predict log K~hydr,(g)~ values for 487 substituted aldehydes having available R-values and 108 substituted aldehydes having available R^+^-values. Good correlations were also found between experimental aqueous phase hydration equilibrium constants (log K~hydr,(aq)~) and summative R/R^+^-values for peripheral substituents on a range of carbonyl derivatives (aldehydes, ketones, esters, and amides), suggesting the structure-reactivity modeling approach can be extended to include all possible combinations of R~1~C(O)R~2~ carbonyl substitution in both gas and aqueous systems. Computationally derived BDEs and Δ~d~H°~(g)~ / Δ~d~G°~(g)~ were in good agreement with the limited experimental and theoretical datasets. BDEs did not generally correlate with any of the Hammett substituent constants or Swain-Lupton parameters considered. Gas phase acidities exhibited high correlation coefficients with Hammett inductive substituent constants (σ~I~) and field effect parameters (F), allowing these to be employed as surrogates for estimating the gas phase aldehydic proton acidities of a larger potential compound range
