Normally either Davies or Güntelberg equation is used in chemical literature to predict the thermodynamic activity coefficients of electrolytes in aqueous solutions when no better equations are available. The validity of these equations and, additionally, of the parameter-free equations used in the Bates- Guggenheim convention and in the Pitzer formalism for activity coefficients were tested with the experimental activity coefficients obtained from the literature data of the following electrolytes at 298.15 K: HCl, HBr, HI, LiCl, NaCl, KCl, RbCl, CsCl, NH₄Cl, NaBr, and KBr. The experimental activity coefficients of these electrolytes can be reproduced within experimental error in the best cases up to a molality of 2.0  mol  kg^-1 by using a two-parameter equation of the Hückel type. The determined Hückel equation is given for each electrolyte considered, and this equation was used in the activity coefficient comparison. The data used in the determination of the Hückel equations covered almost all reliable galvanic cell data available in the literature for dilute solutions of these electrolytes. The results from the present calculations reveal that the parameter-free activity coefficient equations can safely be used in thermodynamic studies only for very dilute electrolyte solutions (i.e. at molalities less than 0.005 mol kg^-1).