The solid phosphoric acid (SPA) catalyzed alkylation of benzene with 1-pentene was studied in the range 160−220 °C, 3.8 MPa, and with aromatic to olefin ratios in the range 6:1−1:6. It could be shown that SPA has the potential to be a selective alkylation catalyst for the production of linear alkyl benzenes (LABs) even at low aromatic to olefin ratios, provided that reaction pathways leading to skeletal isomerization can be suppressed. It was also found that steric effects were more important than electronic effects in governing the alkylation selectivity of SPA and that a protonated cyclopropane structure was the most likely transition-state for alkylation that followed an Eley−Rideal mechanism. Laboratory and pilot plant studies indicated that the carbon chain length of the product was limited to C15, with less than 5% di-alkylation of benzene taking place, even for reactions at 1:6 aromatic to olefin ratio.