Bandgap engineering is an important strategy for tailoring the optical and electronic properties of semiconductor nanocrystals. This work describes the first solution proton alloying process for tuning the bandgap energy of ternary sulfide nanocrystals at room temperature. The proposed strategy circumvents the use of toxic heavy metal ions, while maintaining the size and morphology of the nanocrystals, through a seamless tuning of the bandgap over a wide range. It was shown that proton alloying exhibited different effects on the bandgap energies of ternary sulfide nanocrystals and this could be explained by Density-Of-States (DOS) calculations. Using this approach, enhanced optoelectronic properties of ternary sulfide semiconductor nanocrystals were achieved and proton alloyed ZnIn2S4 showed eight times higher photocatalytic H-2 evolution rate than that of the untreated ones due to increased carrier density and decreased charge transfer resistance. (C) 2016 Elsevier Ltd. All rights reserved.