Diffusion characteristics of volatile organic compounds (VOCs) were investigated indoors using tin oxide gas sensors. The chemicals cause various kinds of symptoms in humans, for example, the sick house syndrome. In this study, eight sensors were installed in a vertical direction and on a plane surface. These sensors were of the same type. The VOC is placed in a generation source, and the sensor output increases as the chemical diffuses. The sensor output becomes higher as the concentration increases. The following chemicals were tried as air pollutants: formaldehyde, toluene, and xylene. The sensor output changes in short, quick steps by slight fluctuations of the wind velocity. Therefore, the differential characteristic of the sensor output was adopted and the noise component was removed as far as possible. A threshold time tth to the characteristic was set up. It is assumed that the examining chemical reaches the installed sensor point in a time greater than this time. The new speed of arrival is proposed using the threshold time. The speed s [cm/min] is indicated using the distance d and the reaching time tth, namely, s = d/tth. Here, d means the distance between the sensor position and the polluting source. As a result, the speed for the sensor that is installed near the ceiling (at a height of 260 cm from the floor) is the highest. And, it became obvious that s was larger for the chemical with a smaller molecule. The speed of formaldehyde for the sensor installed near the ceiling was 700 cm/min and that for the sensor installed at the height of 100 cm from the floor was 370 cm/min. There is almost a two times difference in the speed. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.