In this work, we used firefly oxyluciferin (OxyLH(2)) and its polarity-dependent fluorescence mechanism as a sensitive tool to monitor biomolecular interactions. The chromophores, OxyLH(2), and its two analogues, 4-MeOxyLH and 4,6'-DMeOxyL, were modified trough carboxylic functionalization and then coupled to the N-terminus part of Tat and NCp7 peptides of human immunodeficiency virus type-1 (HIV 1). The photophysical properties of the labeled peptides were studied in live cells as well as in complex with different oligonucleotides in solution. By monitoring the emission properties of these derivatives we were able, for the first time, to study in vitro biomolecular interactions using oxyluciferin as a sensor. As an additional application, cyclopropyl-oxyluciferin (5,5-Cpr-OxyLH) was site-specifically conjugated to the thiol group (Cys-232) of the human protein a-1 antytripsin to investigate its interaction with porcine pancreatic elastase. Our data demonstrate that OxyLH(2) and its derivatives can be used as fluorescence reporters for monitoring biomolecular interactions.