Tetramethylammonium hydroxide (TMAH) is the widely used CMOS compatible anisotropic etchant in silicon wet bulk micromachining to fabricate MEMS structures. 25 wt% TMAH is commonly used because it provides the best etch selectivity between silicon and silicon dioxide with smooth etched surface morphology. If Triton X-100 is used as an additive, it results in significant decrease in the corner undercut while maintaining the good etch rate which is very useful to create mesa structures. This paper mainly focuses on the changes in etching characteristics of 0.1% Triton added 25 wt% TMAH solution when used to fabricate 380 μm deep cavities involving mesa structures in SOI wafer. We observed that the changes in etching characteristics are due to change in the concentration of etching solution which primarily arises because of loss of water content. By replenishing the etching solution with lost water content of same temperature, etching characteristics comparable to that of fresh solution have been obtained. The change in the etchant concentration during the course of etching has been studied. Further, corner undercut and etch rate of silicon have been investigated in both fresh etching solution and the solution replenished with lost water content. This study is especially useful for the control of etching characteristics like the etch rate and undercut during the fabrication of bulk-micromachined structures involving long duration of etching by replenishing the etching solution with the lost water content of same temperature at prefixed intervals of time during the course of etching to regain the etching properties of fresh solution. © 2020 Elsevier B.V.