Older styrene processes based on the chlorination of the side chain of ethylbenzene followed by dehydrochlorination are no longer in use today. The oxidation of ethylbenzene to acetophenone followed by reduction to the carbinol and its dehydration has also decreased in importance; it is now only being practiced in a single plant in Spain.

One stage of the oxidation route, the dehydration of methyl-phenylcarbinol, is still used commercially in anothcr context in a modification of the Halcon process. In this process, involving the indirect oxidation of propene to propylene oxide, ethylbenzene (in its hydropcroxide form) can be used as an auxiliary oxidation system. After being converted into methylphenylcarbinol, it is dehydrated to styrene. About 2.5 kg styrene is obtained per kilogram propylene oxide. Currently, about 15% of the styrene produced worldwide is made with this process.

The main manufacturing route to styrene is, however, the direct catalytic dehydrogenation of ethylbenzene:

One of the first industrial dehydrogenation processes was developed in the IG Farben plant in Ludwigshafen in 1931, and practiced in a 60 tonne-per-year plant. The 'styrene catalysts' were based on three-component systems (ZnO, Al₂O₃, and CaO). The iron oxide catalysts which are generally preferred today were introduced in 1957. They usually contain Cr₂O₃ and potassium compounds such as KOH or K₂CO₃ as promoters. The catalyst is placed in a shell-and-tube reactor, and, in the BASF process, the heat of reaction is supplied externally by a combustible gas. In the USA (e. g., Dow process), the energy for the cleavage is introduced directly by means of superheated steam; here, the fixed-bed catalyst is in a vertical kiln. In this adiabatic mode for the endothermic dehydrogenation, the amount and initial temperature of the steam must be relatively high (2.5-3 kg steam/kg ethylbenzene at ca. 720 °C) to ensure sufficiently high temperatures for the dehydrogenation at the end of the catalyst bed.

A temperature of 550-620°C is necessary for the dehydrogenation. In order to limit side reactions, the partial pressure of ethylbenzene is reduced by admixing an equal amount of steam; this is also done in process with indirect addition of heat. The ethylbenzene conversion is about 40% with conventional catalysts and 60-65 % with modern catalysts (e. g., Shell); the selectivity to styrene is greater than 90%. The byproducts are toluene, benzene, and a small amount of tarry substances. The reaction products are cooled rapidly to prevent polymerization. On reacting 100°C, tar-like substances, then styrene and unreached ethylbenzene are fractionally condensed. The hydrogen formed during the reaction is burned to generate the dehydration temperature. The trend to larger reactors has led to capacities of 70 000 to 100 000 tonnes per year per unit.

After adding a polymerization inhibitor (formerly sulfur, now usually a phenol), the styrene is vacuum distilled. The purification for polymerization applications is very difficult due to the similar boiling points of styrene and ethylbenzene. The required purity of more than 99.8% is reached using four columns.