Activity coefficients for (\mathrm{NaCl}) in aqueous solutions containing a variable molality of (\mathrm{NH}{2} \mathrm{CH}{2} \mathrm{CO}_{2} \mathrm{H}) (glycine) were obtained from the e.m.f.s of the galvanic cell:
[
\mathrm{Na} \text {-glass }\left|\mathrm{NH}{2} \mathrm{CH}{2} \mathrm{CO}{2} \mathrm{H}\left(m{\mathrm{A}}\right), \mathrm{NaCl}\left(m_{\mathrm{B}}\right), \mathrm{AgCl}\right| \mathrm{Ag} \text {, }
]
at different ionic strengths. They were fitted to a modified Pitzer equation by taking into account that the ion-zwitterion interactions depend on both the ionic strength and the zwitterion molality of the solution. Standard deviations for the fittings of about the same size as the accepted experimental uncertainty (\left( \pm 0.10 \mathrm{mV}\right.), i.e. (\pm 0.0008) in (\left.\lg \gamma_{t}\right)) were found by neglecting the ion-zwitterion-zwitterion and ion-ion-zwitterion interactions. Activity coefficients for (\mathrm{NH}{2} \mathrm{CH}{2} \mathrm{CO}_{2} \mathrm{H}) in these aqueous mixtures were thus calculated. They agree well with the literature values obtained for more dilute solutions.