The meta and para β-nitrovinyl derivatives of phenol and benzoic acid have been synthesized. The acidity constant, pKa, was determined for each of these compounds and from these values the following Hammett sigma constants were obtained for the β-nitrovinyl group: σmeta = 0.34, σpara = 0.26, σ− = 0.88. The relationship between these values and the orienting effect of the β-nitrovinyl group on electrophilic substitution is discussed.