Oligomerization of Fischer-Tropsch-derived olefins over a solid phosphoric acid catalyst was studied to improve the quality of the hydrogenated motor-gasoline. It was shown that, with a narrow-cut Fischer-Tropsch butene mixture, rich in 1-butene and low in isobutene, a hydrogenated motor-gasoline with an RON and an MON of 86-88 could be produced at 80% olefin conversion. The influence of propene and pentenes was investigated, and a reactivity sequence for C-3-C-5 mixtures could be established: isobutene > methylbutenes > n-butenes = propene > n-pentenes. It was noted that 1-butene was closer in behavior to isobutene than to 2-butene and had to be treated as such to yield the highest-quality product. It was advantageous to oligomerize 1-butene-rich feed at 160 degreesC or less and to exclude propene and n-pentenes from the reaction mixture. These effects could be explained.