Single-conformation spectroscopy of two diastereomers of 1-(4-hydroxy-3-methoxyphenyl)propane-1,2,3-triol (HMPPT) has been carried out under isolated, jet-cooled conditions. HMPPT is a close analog of coniferyl alcohol, one of the three monomers that make up lignin, the aromatic biopolymer that gives structural integrity to plants. In HMPPT, the double bond of coniferyl alcohol has been oxidized to produce an alkyl triol chain with chiral centers at C(alpha) and C(beta), thereby incorporating key aspects of the beta-O-4 linkage between monomer subunits that occurs commonly in lignin. Both (R,S)- and (R,R)-HMPPT diastereomers have been synthesized in pure form for study. Resonant two-photon ionization (R2PI), UV hole-burning (UVHB)/IR-UV hole-burning (IR-UV HB), and resonant ion-dip infrared (RIDIR) spectroscopy have been carried out, providing single-conformation UV spectra in the S(0)-S(1) region (35200-35800 cm(-1)) and IR spectra in the hydride stretch region. Five conformers of (R,S)- and four conformers of (R,R)-HMPPT are observed and characterized, leading to assignments for all nine conformers. Spectroscopic signatures for alpha-beta-gamma, gamma-beta-alpha, and alpha-gamma-beta-pi chains and two cyclic forms [(alpha beta gamma) and (alpha gamma beta)] of the glycerol side chain are determined. Infrared ion-gain (IRIG) spectroscopy is used to determine fractional abundances for the (R,S) diastereomer and constrain the populations present in (R,R). The two diastereomers have very different conformational preferences. More than 95% of the population of (R,R) configures the glycerol side chain in a gamma-beta-alpha triol chain, while in (R,S)-HMPPT, 51% of the population is in alpha-beta-gamma chains that point in the opposite direction, with an additional 21% of the population in H-bonded cycles. The experimental results are compared with calculations to provide a consistent explanation of the diastereomer-specific effects observed.