The Knudsen mass-loss effusion technique was used to measure the vapour pressure as a function of temperature for trans-cinnamic acid and nine substituted methoxy and dimethoxy cinnamic acids. From the temperature dependence of the vapour pressure the standard molar enthalpies and entropies of sublimation, at 298.15 K, were calculated for all the studied compounds using the estimated value Delta(cr)(g)c(p,m)(o) = -50 J.K-1.mol(-1). The results obtained for trans-cinnamic acid and for the trans-methoxy and trans-dimethoxy cinnamic acids together with literature results for benzoic acid and substituted methoxy and dimethoxy benzoic acids allowed us to derive the correlation: Delta(cr)(g)H(m)(o){T(p = 0.5 Pa)}/(kJ.mol(-1)) = (0.623 +/- 0.026).{T(p = 0.5 Pa)/K} + (-106.3 +/- 9.4) where T(p = 0.5 Pa) is the temperature at which the equilibrium vapour pressure of each crystalline compound is 0.5 Pa.