Properties of the guanidinium cation, GuH(+) = C(NH2 )(3)(+), CAS registry 43531-41-5, were calculated from literature data that were critically examined. The following values are valid for T = 298.15 K and 100 kPa. For the isolated cation (in the ideal gas phase), the molar quantities are: enthalpy of formation, Delta H-f degrees = (462 +/- 3) kJ . mol(-1), enthalpy increment H degrees = H-0 degrees = 14.22 kJ . mol (1), constant pressure heat capacity C-P degrees = 77.89 J . K (1) . mol (1), entropy S degrees = 264.54 J . K . mol(-1), and Gibbs free energy function -(G degrees = H-0 degrees)/T = 216.94 J . K (1) . mol (1). Values for the ion derived from data in aqueous solutions are the radius, r = (0.21 +/- 0.02) nm, the surface charge density sigma = 0.286 C . m (2), and polarizability alpha = 4.44 x 10(-30) m(3). Standard molar values for the aqueous ion are: enthalpy of formation Delta H-f(infinity) = -(140 +/- 7) kJ . mol (1), entropy S-infinity = (201 +/- 7) J . K-1 . mol (1), heat capacity C-P(infinity) = (103 +/- 14) J . K (1) . mol (1), volume V-infinity = (46.3 +/- 3.3) cm(3) . mol (1), electrostriction volume Delta V-electr(infinity) similar to 0, refraction R-D(infinity) = 11.21 cm(3) . mol (1), conductivity lambda(infinity) similar to 39.6 S . cm(2) . mol (1), self-diffusion coefficient D-infinity = 1.06 x 10(-9) m(2) . s(-1), viscosity B-coefficient B-eta = (0.058 +/- 0.006) dm(3) . mol (1), dielectric decrement de(r)/d(c) = -10.0 dm(3) . mol (1) (chloride salt), and surface tension decrement d(gamma)/d(c) = (0.04 +/- 0.20) dm(3) . mol (1). Standard molar values derived from a combination of the data for the gaseous and aqueous ions are the enthalpy of hydration Delta H-hydr(infinity) = -(602 +/- 8) kJ . mol (1), entropy of hydration Delta S-hydr(infinity) = -63.7 J . K (1) . mol (1), and the structural entropy Delta S-struct(infinity) = 83 J . K (1) . mol (1). (C) 2011 Elsevier Ltd. All rights reserved.