Described in detail in this paper is a useful method which can be used to study avalanches in electronegative gases and then to determine electron swarm parameters (ionization coefficient α, attachment coefficient η, and drift velocity v) including the number and distribution of primary electrons released from the cathode of a parallel electrode configuration by high energy laser pulses. The method has been used to study electron swarm parameters in mixtures of SF6 and CO2. Relations between the parameters (α/P, η/P and ν) and E/P (E=electric field, P=gas pressure) are given. A number above 108 of primary electrons has been obtained. The paper describes also the measurement system used in the study and improvements of the system that can be made.