It has become increasingly apparent that basic information regarding the interaction of the relevant gaseous species at the diamond surface is important for a better understanding of the chemical vapor deposition (CVD) process. The non-linear phenomena of second-harmonic generation (SHG) and sum frequency generation (SFG) have the potential of probing the surface in situ during diamond CVD. We have initiated a program of investigating the low index planes of the diamond single crystal using conventional surface science techniques in conjunction with SHG and SFG measurements. As a first step in this effort, Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy, spectroscopy and low energy electron diffraction (LEED) temperature-programmed desorption have been used to study the interactions of H and CHx species with the reconstructed diamond C(111) (2 × 1) surface. It is found that a very small quantity of chemisorbed H atoms (0.05 ML (monolayer) or less) can induce the (2 × 1)-to-(1 × 1) phase transformation and uphold the sp3 structure up to nearly 1400 K. SHG measurements during the exposure of the surface to hydrogen showed direct correlation with observations by LEED and AES. The vibrational spectrum of the surface hydrogen is obtained using IR visible SFG. On a fully relaxed (1 × 1) surface a single sharp peak at about 2830 cm-1 is observed which can be identified as the CH stretching mode from H on top sites. The SFG spectrum after exposure of the (2 × 1) surface to methane through a hot filament shows the dominant adsorbed species to be the methyl group.