Pure and ZnS added ADP (ammonium dihydrogen phosphate) single crystals have been grown at room temperature by the free evaporation method. A total of six crystals have been grown and characterized structurally, chemically, thermally, optically, mechanically and electrically by using the suitable standard methods. Results obtained indicate that the impurity molecules have entered into the ADP crystal matrix. ZnS addition is found to increase the SHG efficiency significantly. All the grown crystals exhibit good optical transmission in the entire visible region. Results of AC and DC electrical measurements indicate a normal dielectric behaviour for all the six crystals grown. The electrical parameters, viz., DC electrical conductivity, dielectric constant, dielectric loss factor and AC electrical conductivity are found to increase with the increase in temperature. The optical, mechanical and electrical parameters are found to vary nonlinearly with the impurity (ZnS) concentration.