The backscatter information available from many modern multibeam echosounder systems (MBES) has been shown to be useful for a number of purposes.  Linearity of the system response is posited to be an important requirement for many backscatter processing techniques.  A procedure to measure the system linearity is developed for the Reson 7125.  These measurements are performed both in a controlled test tank environment and with systems installed on operational platforms.  The linearity of the system with respect to power, gain, and the returned signal level is evaluated.  We determine that it is possible to drive the system to nonlinear behavior.  The consequences of nonlinearity on both bathymetric measurements and backscatter intensity values are developed theoretically and tested against experimental observations. We find that nonlinear performance generally complicates and degrades both backscatter and bathymetric data products.