A technique for identifying a vein in a patient is disclosed. The technique involves the application of a thermochromic ink to a venous area on the skin of a patient (e.g., human or animal). The thermochromic ink comprises thermosensitive color-changing microcapsules that contain a proton-accepting chromogen and a desensitizer. The desensitizer possesses a melting point above which the chromogen is capable of becoming protonated, thereby resulting in a color change. Thereafter, the venous area is observed for the color change.