Batch　tests　were　employed　to　estimate　the　optimal　conditions　for　photocatalytic　degradation　of　tetracycline　using　In2S3　under　natural　solar　radiation　by　response　surface　methodology.　Three　key　operating　parameters,　such　as　dosages　of　In2S3,　initial　concentration　of　tetracycline　and　an　initial　pH　value,　were　selected,　and　their　interrelationships　were　studied　by　the　Box-Behnken　design.　The　experimental　data　and　ANOVA　analysis　showed　that　the　coefficient　of　determination　(R-2)　was　0.9998,　which　demonstrated　that　the　model　was　significant.　The　optimum　conditions　were　predicted　to　give　a　maximal　removal　efficiency　of　100%　at　the　In2S3　dosage　of　2.5　g/L,　an　initial　tetracycline　concentration　of　20　mg/L　and　pH　of　7.0.　The　prediction　was　tested　by　triplicate　experiments,　where　a　removal　efficiency　of　100%　was　achieved　under　the　chosen　optimal　conditions.　This　excellent　correlation　between　the　predicted　and　measured　values　provided　confidence　in　the　model.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.