The ability to control the molecular structure (e.g., degree of branching) of iso-olefins produced from oligomerizing light olefins is a valuable tool for tuning the final compositions of hydrocarbon fuels and targeting specific fuel properties. In this study, we demonstrated that the degree of branching of iso-olefins obtained from butene oligomerization can be controlled by tuning process conditions (i.e., temperature, weight hourly space velocity WHSV, nature of butene feedstock) and by choosing the proper catalyst (i.e., Amberlyst-36 vs. Y/ZSM-22). We produced three types of iso-olefin mixtures: 1) a methyl-heptenes rich (74 wt%) mixture,2) dimethyl-hexenes rich (80–96 wt%) mixtures, and 3) highly branched (i.e., ≥3 methyl substitutions) iso-olefins rich (>50 wt%) mixtures. While dimethyl-hexenes are preferentially formed at lower temperatures (60–100 °C) and WHSV (i.e., 2 hr−1), methyl-heptenes are favorably produced at higher temperatures (>100 °C) and WHSVs (i.e., 7 hr−1) over Amberlyst-36. The use of either 1-butene or 2-butene as feedstock resulted in liquid products with similar branching because facile intramolecular isomerization occurs prior to oligomerization. The use of isobutylene feedstock forms a more branched olefin product. For each type of iso-olefins mixture, we determined fuel properties including research octane number (RON), motor octane number (MON), and octane sensitivity (S), which is the difference between RON and MON. We found that not only RON and MON but also S values increased with the degree of branching of these complex mixtures of iso-olefins. The highest RON of 99.7 and S value of 9.3 were obtained for a mixture of highly branched iso-olefins.