A system and method are provided for in-line processes of blending butane into gasoline streams, and for blending butane into a gasoline stream at any point along a petroleum pipeline. The invention additionally provides a method for measuring the vapor pressure and vapor to liquid ratio of the gasoline, both upstream and downstream of the blending operation, as well as the sulfur content of the butane entering the blending operation. The blending operation can be controlled to ensure that the blended gasoline meets EPA requirements for vapor pressure and sulfur content of gasoline. The invention further provides a method for accessing and monitoring the operation off-site.