In this communication an investigation is reported in which ion-pair formation is sensed quantitatively by the laser-Raman technique using as the model electrolyte system solutions of AgNO3 in acetonitrile. The measurements were extended to the dilute-solution range in an attempt to intercompare the results forKa with those calculated from conductance studies and the Fuoss-Onsager theory of conductance. The highest dilution attained in the spectroscopic studies was in the region of 10⩼−3M. In the region of concentration overlap, it is found that the values forKa from the present laser-Raman studies and from the Fuoss-Onsager theory of conductance are, respectively, 84±14 and 70.3±1.2; corresponding values for the ion-size parameteraj fall in the range of 3.1–4.0 A. The latter are examined relative to the predictions spectroscopically for contact-ion pair formation at these high dilutions.