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ever, in t fails to d many-bo solvent d of inform be gained rences trome-ission raight-erence g thatedge of the time dependence of the echo emission. In previous work, time resolution was achieved by record-ing the upconverted echo signal as a function of a time delay with respect to an additional gating pulse.2 – 4 Recently we suggested that it would be both easier and more comprehensive to measure the echo through Fourier-transform spectral interferometry (FTSI),5 a technique based on spectral interferometry,6 – 8 which yields the amplitude and the phase of a coherent tran-sient emission, provided that an adequate reference pulse is available. In this Letter, we report on the experimental demon-stration of FTSI FWM by measuring in the frequency the reference pulse is transform limited, we can deduce the amplitude and the phase of the echo, yielding a complete measurement of the electric f ield. This as-sumption of a chirp-free reference pulse is based on the excellent agreement between the measured and the computed autocorrelations of the laser pulses, for which the calculation is based on the measured laser spectrum with a spectral phase set to zero (the inset of Fig. 1). The availability of clean transform-limited pulses directly delivered by the oscillator makes un-necessary here the use of more complicated phase-measurement techniques.14 Finally, we mention that, although phase locking should be used in this setuphe condensed phase, the Bloch equation often escribe the system properly, e.g., because of dy effects in semiconductors2,3 or to complex ynamics in liquids.4 An invaluable amount ation about such complicated processes will in a photon-echo experiment from the knowl-through a beam splitter, and the spectral interfe between the two beams are recorded in a spec ter. The phase difference between the FWM em and the reference can then be extracted with st forward Fourier transforms, as long as the ref pulse precedes the FWM emission.5,14 AssuminMeasurement of photon ec Fourier-transform s