We have experimentally investigated three types of two-color techniques for the detection of the hydroxyl radical using an infrared laser to pump individual ground-state rovibrational transitions and an ultraviolet laser to probe electronic transitions from the pumped intermediate levels. Signal scaling relations and saturation studies are compared for the following techniques: two-color laser-induced fluorescence (TC-LIF), polarization spectroscopy (TC-PS), and resonant four-wave mixing (TC-RFWM). Collisional effects are observed, but the time resolution set by the 1.5-ns laser pulses is not sufficient to determine state-to-state energy transfer rates in an atmospheric pressure flame. (C) 2003 Elsevier Science B.V. All rights reserved.