An instrument is described which was designed for precision studies of isothermal diffusion in liquids by optical methods using interference fringes. This optical diffusiometer is described in some detail because it has been found to function exceptionally well for such studies and also because its main components and general design allow it to be adapted to a wide variety of optical measurements and other studies. It is basically a large precision optical bench. An 884 cm long steel beam serves as the rigid base on which are bolted stainless steel dovetail ways which extend along the full length of the beam; the contact surfaces of these ways are straight and flat to about 0.005 cm over their entire length. Riders which support the optical components may be clamped at any position along the 884 cm length of the ways. The riders are designed so that components mounted on them may be centered on and made perpendicular to the optic axis of the instrument. A special measuring rider is used in conjunction with measuring notches cut every 30.48 cm (12 in.) in one edge of the ways in order to determine the position along the ways of any other rider with an accuracy of about 0.002 cm. The entire instrument was designed with special emphasis on versatility, rigidity, precision, ease of adjustment, and freedom from vibration. This instrument permits diffusion coefficients to be measured with a greater precision than has been possible with previous models.