A method and apparatus are provided for monitoring an immersion photolithography process, the method including supplying an immersion fluid having an initial refractive index, performing photolithography using the supplied immersion fluid, recovering the used immersion fluid; and the apparatus including a light source, one or more fluid passageways disposed relative to the light source, and a light detector disposed on an opposite side of the fluid passageways relative to the light source for measuring a refractive index of a fluid in the fluid passageways.