A self‐nulling ellipsometer was used to follow the optical changes which occur when galvanostatic anodic oxidation of tungsten in 0.5M sulfuric acid is interrupted at 5V intervals by the application of cathodic transients which color the oxide film. The film undergoes two distinct optical changes during the cathodic cycle, the first due to the strain and anisotropy resulting from electrostriction, and the second due to the optical absorption associated with electrochromism. For color cycles of limited duration, application of an anodic current will bleach the film with little optical hysteresis and return it to its initial thickness and refractive index. Longer color cycles reach a point where further coloring is accompanied by film dissolution. The optical data fit a model in which coloring takes place by a progressive change in the entire film rather than by movement of an interface separating colored and bleached layers of a duplex film. The coloring process appears to have no clearly defined end point either in sulfuric acid or in acetic acid where the color cycle can be extended without film dissolution.