For ducts operated as MHD flowmeters, it is demonstrated that the sensitivities are substantially affected by variable fluid viscosity and electrical conductivity. The analysis considers a mathematical model based on fully developed laminar MHD flows with constant heat flux boundary conditions. By considering the pertinent equations in dimensionless form and assuming a power law dependency of the fluid properties on temperature, flowmeter sensitivities are calculated as functions of the Hartmann number, duct aspect ratio, and the magnitude of the property variations. It is shown that, even for small property variations, the flowmeter sensitivities deviate significantly from constant property values. For larger variations, they can be as much as 60% below ideality for heated walls and 30% above for cooled walls.