This paper studies the UV-vis absorbance of colloidal gold nanoparticles at 400 nm and validates it as a method to determine Au(0) concentrations in colloidal gold solutions. The method is shown to be valid with restrictions depending on the investigated system. The uncertainty of the determined Au(0) concentration can be up to 30%. This deviation is the result of the combined influence of parameters such as particle size, surface modification, or oxidation state. However, quantifying the influence of these parameters enables a much more precise Au(0) determination for specific systems. As an example, the reduction process of the well-known Turkevich method was monitored and the Au(0) concentration was determined with a deviation of less than 5%. Hence, a simple, fast, easy, and cheap in situ method for Au(0) determination is demonstrated that has in the presence of other gold species such as Au(III) an unprecedented accuracy.