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We present a design for the detection of relaxation kinetics by midinfrared transient spectroscopy. Spectra can be collected with 150 ns time resolution after a single pump pulse. This eliminates the need for flowing samples and the shot-to-shot background noise common in pump–probe spectroscopy. Difference frequency generation from a Ti:sapphire oscillator with a dual-wavelength profile produces broadband femtosecond IR pulses tunable below 1800 cm-1 at 91 MHz repetition rate. This pulse train probes a sample by direct absorption, followed by dispersion of the 200 cm-1 wide spectra in a double-prism spectrometer onto a fast multichannel mercury cadmium telluride detector. In our setup, sample relaxation is initiated by a single temperature jump. We present the fast folding dynamics of the protein λ repressor as an application. © 2004 American Institute of Physics.