Disclosed are a method and an apparatus for precision optical measurement of solute concentration and temperature of a transparent liquid. The apparatus includes a vessel (12) which contains a sample of liquid (14), the refractive index of which is to be determined, a light source (22) and a beam position sensor (30). The vessel (12) has an entrance side and an exit side, with a known angular relationship therebetween, and is situated in a known ambient The light source (22) is located so as to cause a beam of light to impinge on the entrance side at an angle with respect to a normal to an outer surface of the entrance side, to pass through the liquid (14), and to then pass through the exit side, from which it exits at an angle with respect to a normal to an outer surface of the exit side. The beam position sensor (30) provides data for determining the exit angle thereby permitting the refractive index of the liquid to be determined, based on the angles.