The determination of biological thiols is considered to be of great interest because of the participation of these molecules in physiological disorders as indicators of their existence and progress, such as Alzheimer's disease, cancer, and cystic fibrosis, among others. Optical techniques for the quantification of these analytes are superior to other techniques in terms of selectivity, sensitivity, and cost. We describe here a simple protocol to determine biological thiols by means of a colorimetric analysis based on a magenta color interaction with 5,5'-dithiobis(2-nitrobenzoic acid). This methodology leverages the technology developed in smartphones, and it can be transferred to a simple analytical procedure through the use of absorbent strips and a cell phone. Excellent characteristics were observed, and this method opens up alternative avenues for the detection of biological thiols in a very simple and portable manner.