In order to obtain a more uniform and reliable product from burner reactors, there is provided a control for the burner (18) which includes, a sensor (10) to continuously monitor a parameter of the product, suitably temperature, a comparator (24) to output an electrical signal representative of the deviation of the parameter from a reference value, means such as an A/D converter (26) to latch digital representations of the signal amplitude at set intervals and means (30, 34, 35) to adjust the burner (18) in dependence upon the presently latched digital representation and upon the direction of the deviation represented thereby.