The lipid chain conformation predicted by a recent molecular dynamics simulation (MD) of the fluid and gel phases of a dipalmitoylphosphatidylcholine (DPPC) phospholipid bilayer is described in detail and, where possible, is compared to the results of IR measurements. Conformational statistics for the MD simulated fluid-phase chains are compared with those for an unconstrained liquidlike n-C16H34 system estimated using a rotational isomeric state model. The constraints imposed on the bilayer chains are found to reduce the concentration of gauche bonds, suppress sterically unfavorable conformational sequences, and increase trans/gauche bond segregation. The concentrations of short (1-3 bond) conformational sequences for the MD-simulated bilayer are found to be nearly the same as those for the unconstrained RISM n-C-16 system with the same gauche bond concentration as the constrained chains. For long sequences, however, especially long all-trans sequences, there are large differences between the systems. The bilayer-normal distribution of CC bonds across the gel phase bilayer is found to be largely determined by longitudinal chain displacement, whereas for the fluid phase, the distribution is determined by conformational disorder. Order-disorder heterogeneity is observed in the MD-simulated fluid-phase bilayer in the form of a low concentration of more or less well-defined, highly transient microdomains consisting of the order of 20 all-trans or nearly all-trans chains. The packing of these chains is similar to that found for the simulated gel phase, Both chain-tilt and chain order/disorder correlations between opposing monolayers were found. The two most reliable IR methods for determining chain conformation are critically reviewed. Previously reported IR measurements on the gel- and fluid-phase DPPC bilayer using these methods are reinterpreted. The distribution of gauche bonds in the gel is found to be determined primarily by longitudinal displacements of the chain. By combination of the measured concentrations of bond-pair conformations tt, gt, and gg in the fluid phase, the overall concentration of gauche bonds is estimated to be 0.14 +/- 0.04. This value is significantly lower than most previously reported values and much lower than the 0.28 derived from the MD simulation.