Numerical and experimental investigations of mixing in pipelines with side and opposed tees are carried out. Cold water flowing in a main pipe is mixed with hot water flowing through a tee. The temperature is measured experimentally to quantify the degree of mixing. The velocity and temperature fields are also solved numerically. The effects of the mesh size, mesh-localized refinement, dependence of the fluid physical properties on temperature, and turbulence model on numerical results were examined. Experimental results show good agreement with corresponding predictions of the numerical model over a relatively wide range of Reynolds number; however, close agreement is harder to obtain in the vicinity of the jet through the tee. The pipe length required to achieve 95% mixing is found to be a function of U-j/U-m. The angle at which the side jet is injected determines whether the jet impinges on the opposite wall and also affects the pipe length required to achieve 95% mixing. This work recommends that industry should not use 90degrees tees because of possible poor mixing at certain velocity ratios and hard impingement. For pipe diameters ranging from 1 to 16 in., if d(j)/d(m) is kept constant, then for any velocity ratio the 95% mixing is achieved at a distance corresponding to about the same number of the mainpipe diameters. For opposed jets, numerical convergence was harder to obtain at high Reynolds numbers. Some modifications, including the staggering of the two jets, made it easier for the solution to converge.