A process for controlling downstream water concentration in an acetic acid production process comprising: contacting methanol and carbon monoxide in the presence of a reaction medium under carbonylation conditions sufficient to form a carbonylation product comprising acetic acid, wherein the reaction medium comprises a carbonylation catalyst, water in an upstream water concentration of from 1 wt.% to 14 wt.% water, and a tertiary phosphine oxide; recovering acetic acid from the carbonylation product; and controlling a downstream water concentration by determining a target water concentration and introducing the tertiary phosphine oxide to the reaction medium at a rate, basicity, concentration or combination thereof sufficient to provide a downstream water concentration within 1 wt.% of the target water concentration: wherein the recovering acetic acid comprises: flashing the carbonylation product to form a vapor fraction and a liquid fraction; separating the vapor stream to form an overhead stream, an acetic acid stream and a bottoms stream; and drying the acetic acid stream to remove water therefrom; and wherein the tertiary phosphine oxide is a compound mixture of at least four phosphine oxides, where each phosphine oxide has the formula OPX 3, wherein O is oxygen, P is phosphorous and X is independently selected from alkyls; wherein the downstream water concentration is selected from a concentration of water in the carbonylation product, the vapor fraction, the acetic acid stream or combinations thereof; wherein the upstream water concentration is from 2 wt.% to 6 wt.%; wherein the target water concentration is greater than the upstream water concentration; and wherein the target water concentration is from 4 wt.% to 15wt.%. contacting methanol and carbon monoxide in the presence of a reaction medium under carbonylation conditions sufficient to form a carbonylation product comprising acetic acid, wherein the reaction medium comprises a carbonylation catalyst, water in an upstream water concentration of from 1 wt.% to 14 wt.% water, and a tertiary phosphine oxide; recovering acetic acid from the carbonylation product; and controlling a downstream water concentration by determining a target water concentration and introducing the tertiary phosphine oxide to the reaction medium at a rate, basicity, concentration or combination thereof sufficient to provide a downstream water concentration within 1 wt.% of the target water concentration: wherein the recovering acetic acid comprises: flashing the carbonylation product to form a vapor fraction and a liquid fraction; separating the vapor stream to form an overhead stream, an acetic acid stream and a bottoms stream; and drying the acetic acid stream to remove water therefrom; and wherein the tertiary phosphine oxide is a compound mixture of at least four phosphine oxides, where each phosphine oxide has the formula OPX 3, wherein O is oxygen, P is phosphorous and X is independently selected from alkyls; wherein the downstream water concentration is selected from a concentration of water in the carbonylation product, the vapor fraction, the acetic acid stream or combinations thereof; wherein the upstream water concentration is from 2 wt.% to 6 wt.%; wherein the target water concentration is greater than the upstream water concentration; and wherein the target water concentration is from 4 wt.% to 15wt.%. flashing the carbonylation product to form a vapor fraction and a liquid fraction; separating the vapor stream to form an overhead stream, an acetic acid stream and a bottoms stream; and drying the acetic acid stream to remove water therefrom; and wherein the tertiary phosphine oxide is a compound mixture of at least four phosphine oxides, where each phosphine oxide has the formula OPX 3, wherein O is oxygen, P is phosphorous and X is independently selected from alkyls; wherein the downstream water concentration is selected from a concentration of water in the carbonylation product, the vapor fraction, the acetic acid stream or combinations thereof; wherein the upstream water concentration is from 2 wt.% to 6 wt.%; wherein the target water concentration is greater than the upstream water concentration; and wherein the target water concentration is from 4 wt.% to 15wt.%.