The design and development of new pyrene-based fluorescent probes, P-Hcy-1 and P-Hcy-2, which display selective fluorescence enhancements in response to homocysteine (Hcy), are described. The distinctly different fluorescence responses of P-Hcy-1 and P-Hcy-2 to Hcy vs. Cys are explained by theoretical calculations. Finally, the results of cell experiments show that these probes can be used to selectively detect Hcy in mammalian cells.