The massic energy of combustion of crystalline acetylsalicylic acid was determined in an NIST adiabatic-aneroid-rotation bomb calorimeter The standard molar enthalpy of combustion at T = 298.15 K and p(o) = 10(5) Pa is Delta(c)H(m)(o)(C9H8O4, cr, 298.15 K) = -(3926.71 +/- 0.34) kJ . mol(-1) The derived standard molar enthalpy of formation is Delta(f)H(m)(o)(C9H8O4, cr, 298.15 K) - (758.19 +/- 0.52) kJ . mol(-1). The molar enthalpy of fusion Delta(fus)H(m)(o)(C9H8O4, cr, 414.0 K) = (29.80 +/- 0.22) kJ . mol(-1) was determined by differential scanning calorimetry. The molar Kirchhoff relationship, d(Delta H) = Delta C(p)dT, was used to convert this value to T = 298.15 K, Delta(fus)H(m)(o)(C9H8O4, cr, 298.15 K) = (19.28 +/- 0.22) KJ . mol(-1). The molar enthalpy of vaporization was estimated using the group contribution scheme of Lebedev and Miroshnichenko. The molar enthalpy of vaporization of acetylsalicyclic acid was estimated as Delta(vap)H(m)(o)(C9H8O4, cr, 298.15 K) = (106.4 +/- 2.0) kJ . mol(-1) The molar enthalpies of fusion and vaporization were used to calculate enthalpies of formation for the liquid and gaseous states of acetylsalicyclic acid, Delta(f)H(m)(o)(C9H8O4, 1, 298.15 K) = -(738.91 +/- 0.56) kJ . mol(-1) and Delta(f)H(m)(o)(C9H8O4, g, 298.15 K) = - (632.5 +/-2.1) kJ . mol(-1), respectively.