Effects of pyrolysis temperature (300-1000 degrees C), heating rates (100, 500, 1000, and 10,000 degrees C/s), and particle sizes (53-63, 104-120, 177-270, and 270-500 mu m) on the yields and formation rates of tar, light oils, total gases, and char from pyrolysis of beech-wood under 1 arm helium pressure were studied. Wood particles were pyrolyzed in strips of stainless steel wire mesh in a captive sample apparatus; and yields of products were measured in weight percent of original wood as a function of temperature for different heating rates and particle sizes. The overall weight loss achieved from pyrolysis of this wood was about 90%. The total yields of tar and light oils from pyrolysis of this wood accounted for up to 80% of the original wood above 400 degrees C. Due to the post-pyrolysis reactions of tar and light oils, the tar and light oils yields go through a maximum with pyrolysis temperature for all particle sizes and most heating rates studied here. As particle size increases from 53-63 mu m to 270-500 mu m the maximum tar yield decreases from 53% to about 38%. The maximum tar yield also decreases with increasing the heating rate from 70% at 100 degrees C/s to 48% at 10,000 degrees C/s heating rate. Theses results indicate that as the intra-particle post-pyrolysis cracking reactions of tar increases at higher heating rates and with larger particles the tar yield decreases. Tar was also analyzed with GPC for the effects of above pyrolysis parameters on the tar molecular weight. The tar average molecular weight. remains relatively constant (M(W) = 300 amu, M(n) = 155 amu, and M(Z) = 483 amu) under helium atmosphere with pyrolysis temperature at 1000 degrees C/s heating rate and with 53-63 mu m particle size. The average molecular weight of tar does not significantly varies with heating rate, but it decreases as the particle size increases.