Changes in yields and structural characteristics of tars/oils produced in the first stage of a two-stage fixed-bed (＇hot-rod＇) reactor were determined as a function of sample configuration and pressure. The work included a critical evaluation of analytical methods for monitoring structural changes in biomass-derived liquids. Total volatile and tar/oil yields decreased and significant structural changes were observed with increasing sample bed height and pressure, leading to ＇lighter＇ tars/oils. Products were observed to become more aromatic and less oxygenated. Less intense degradation reactions were observed under hydrogen than under helium. 1-Methyl-2-pyrrolidinone (NMP) was used as eluent in size-exclusion chromatography; solubility limitations were found with tetrahydrofuran as eluent. Good agreement was obtained between structural changes inferred from size-exclusion chromatography and u.v. fluorescence and u.v. absorption spectroscopies. The characterization of liquid products by H-1 n.m.r. was found to be less informative than expected, due to the multiplicity of structural assignments in relatively narrow bands. The work suggests that mild hydropyrolysis in Peeper beds may be considered as a process step more likely to produce lighter, less oxygenated and more stable tars/oils than liquids produced at atmospheric pressure. However, the reforming process achieved by extraparticle reactions and by the effect of pressure gave rise to significant loss of liquid product.