In ultralow-temperature HPLC, analyte retention is often enhanced, inhibiting elution. To solve this problem, we have investigated the use of low-molecular-weight hydrocarbons, methane and ethane, as the mobile phase in a monolithic ODS column. Analyte retention was successfully reduced by the use of these mobile phases, and elution of mono-and di-chloromethane and n-octane, which were not eluted in our previous work using a liquid nitrogen based mobile phase, was achieved. The analysis of octane structural isomers revealed that, in cryogenic HPLC, the retention of branched octanes was significantly reduced compared to the retention of n-octane, i.e., the retention factor of iso-octane (2,2,4-trimethylpentane) was almost negligible. The retention factors of branched octanes were distributed between those of n-pentane and n-heptane in HPLC at-176°C, whereas, in gas chromatography at 50°C, these values were between those of n-heptane and n-octane.