New synthesis routes have been investigated for the production of dinitrotoluenes (DNTs) from toluene or an equimolar mixture of 2-nitrotoluene (2-NT) and 4-nitrotoluene (4-NT) and nitric acid using solid acids as heterogeneous catalysts. The main objective was the replacement of liquid sulphuric acid. A secondary goal was the increase of 4-NT and 2,4-DNT in the product mixture, which are more valuable products. Particular consideration was given to the feasibility of future large-scale process implementation. Continuous flow vapour phase reaction, and liquid phase reaction with simultaneous distillation were investigated. DNT formation in the vapour phase reaction was negligible with all solid acids tested, Preshaped silica impregnated with sulphuric acid was the most active catalyst but a continuous loss of sulphuric acid with time-on-stream was observed. Zeolite beta provided a higher 4-NT to 2-NT ratio than ZSM-5, ZSM-12 and mordenite, but deactivated after 5-10 h on-stream, Regeneration of beta was possible by thermal treatment. The para-selectivity of mordenite could be improved by the creation of a mesoporous system. In the reaction with simultaneous distillation, supported liquid acids exhibited true catalytic behaviour for the conversion of NT to DNT, but a loss of the impregnated acid was observed. Zeolite beta was about half as active as supported liquid acids and gave an exceptionally high 2,4-DNT selectivity of up to 94%, compared to 74-79% obtained with all other solid acids.