We present a new technique of broad-band heterodyne-detected sum frequency generation (HD-SFG) spectroscopy and demonstrate its high sensitivity allowing surface-selective measurements of vibrational spectra at submonolayer surface coverage, as low as a few percent of a monolayer. This was achieved without the help of surface enhancement phenomena, on a transparent dielectric substrate (water), and without introducing fluorescent labels, in fact, without utilizing any electronic resonances. Only the intrinsic vibrational transitions were employed for the detection of the analyte molecules (1-octanol). Unlike conventional (homodyne-detected) SFG spectroscopy, where the signal intensity decreases quadratically with decreasing surface coverage, in HD-SFG, the scaling is linear, and the signal is amplified by interference with a reference beam, significantly improving sensitivity and detection limits. At the same time, HD-SFG provides the phase as well as the amplitude of the signal and thus allows accurate subtraction of the non-resonant background-a common problem for surfaces with low concentrations of analyte molecules (i.e., weak resonant signals).