Numerical and experimental investigations of mixing in pipelines with side-tees were carried out to optimize mixing in such pipelines. Side-tees comprise a side pipe of 0.003175 m and 0.00635 m in diameter connected to a 0.0254 m main pipe were used in this study. The tip of the side pipe protrudes to the centre line of the main pipe. Hot water introduced through the side-tee was mixed with cold water flowing in the main pipe. Temperature was measured experimentally to quantify the degree of mixing. Numerically, the temperature field was computed and then compared with experimental results to validate the models. The length of the main pipe downstream of the jet inlet required to achieve 95% mixing was then determined. Numerical results showed good agreement with experimental results. The pipe length required to achieve 95% mixing is a strong function of the ratio of the side-stream to mainstream velocities. The angle of the jet has a significant effect on mixing length. Mixing depends on the flow patterns created by the jet impingement. These patterns depend on the degree of penetration of the jet in the main flow stream.