The excellent thermal stability of tetrahydroquinoline (THQ) under reducing conditions [1] has led to its use as a slurry liquid for several catalytic reactions: the synthesis of methanol over "zinc chromite" catalyst [21, the synthesis of higher alcohols over promoted "zinc chromite" [31, and the dehydrogenation of methanol to formaldehyde over various copper-containing catalysts [4,5]. However, the rate and selectivity of alcohol synthesis over zinc chromite catalyst was much different with THQ as the slurry liquid than with several similar compounds. It also was found that THQ was alkylated during both alcohol synthesis and methanol dehydrogenation. To understand the behavior of THQ-derived slurry liquids, various analyses were carried out on a sample of this liquid that was obtained after 240 h of continuous operation under methanol synthesis conditions. Silica gel liquid chromatography (LC) and high performance LC (HPLC) were used to fractionate the "spent" slurry liquid, while gas chromatography/mass spectroscopy (GC/MS), Fourier transform infrared (FT-IR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy were applied to identify the major compounds. Methyl-, dimethyl-, and trimethyl-THQ comprised more than 80% of the "spent" slurry liquid. The balance primarily was various methylated indoles. A methyl group always was attached to the N atom in the ring structure. There was no evidence of further alkylation of methyl groups, These results appear to eliminate the possibility that the observed differences between THQ and similar hydrocarbon slurry liquids result from the nucleophilicity of secondary amines in the liquid. They also suggest that alkylation of THQ will eventually stop as the ring positions in THQ become saturated. A mechanism for the alkylation of THQ is proposed.