Effects of cesium related reactions are investigated using a simulation code forH−ion sources. Effects begin to appear when cesium density is1011 cm−3,but are still small and become large when the cesium density is greater than1012 cm−3.TheH−density decreases due to electron detachment. Decreasing the plasma potential by cesium seeding results in 12&percent; smallerH−density. The minimum of the plasma potential shown experimentally by Bacal is found to correspond to a cesium coverage of 50&percent; with the use of Langmuir adsorption isotherm. Surface production is effective only when the cesium density is around1011 cm−3.For cesium density greater than1012 cm−3,the effect of the cesium related volume reaction becomes larger. ©1998 American Institute of Physics.