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The streamer zone of positive leader during the breakthrough phase of long sparks was experimentally investigated with two methods. One of the methods is the analysis of streamer-zone images obtained with a high-speed framing camera with image enhancement. This method allowed us to estimate the spatial distribution of streamer density and low-frequency conductivity in the streamer zone. The other method is the microwave probing, which we applied for the first time to long sparks. The attenuation of microwave beam in the streamer zone in our experiments is proportional to the total number of free electrons inside the microwave beam. Experimental data on the microwave attenuation combined with the streamer density found using the first method allowed us to estimate the total number of free electrons in one streamer. The following parameters were obtained. The streamer density in the center of the streamer zone is (0.6-1) . 10(5) m(-3), and the total number of streamers in the streamer zone is 4 . 10(5)-10(6). The average total number of free electrons in one streamer is about 3 .10(10). Low-frequency conductivity on the axis of streamer zone was estimated to be typically 2 . 10(-5) S/m, which is similar to that estimated for corona sheath in lightning. Both methods are based on the assumption of constancy of electric field and similarity of all streamers inside the streamer zone. The overall results of this study are generally consistent with this assumption.