Maleimide-thiol chemistry is widely used for preparing nanoparticles (NPs) conjugates. The surface modification of nanoparticles with free sulfhydryl group is a common strategy to activate nanoparticles. A great challenge has been the quantitation of thiol groups on the thiol-modified surface. Beadlite™ Rapid Colorimetric Thiol Quantitation Kit provides a robust method to quantify thiol-functionalized nanoparticle with our proprietary Thiolite™ 490 that has its maximum absorbance at ~490 nm. The kit quantifies sulfhydryl groups by first reacting nanoparticles with a known amount of Thiolite™ 490 present in excess and the remaining unreacted Thiolite™ 490 is measured by its absoprtion. The amount of sulfhydryl group on the NP surface is calculated to be the difference between the initial amount of Thiolite™ 490 and the amount of unreacted Thiolite™ 490. Beadlite™ Rapid Colorimetric Thiol Quantitation Kit can be performed in a traditional cuvette, NanoDrop™ Spectrophotometer or a convenient 96-well absorbance plate reader. This kit has been widely used for the rapid quantification of sulfhydryl groups on the surface of latex beads, agarose beads, or magnetic nanoparticles.