The effects of cations on the yields of char, tar, light oils, and total gases from rapid pyrolysis of beech wood were studied. Raw wood, acid washed wood, and wood impregnated with potassium, sodium, and calcium cations were pyrolyzed in 1 atm pressure of helium at 1000 degrees C s(-1) heating rate to a peak temperature of approximate to 1000 degrees C. Experiments were carried out in an electrical screen heater reactor, and the yields of products were determined as a function of pyrolysis peak temperature. Acid washed wood samples gave the highest tar yield (about 61% by weight of the original wood), whereas wood samples impregnated with potassium or sodium cations gave the lowest yield of tar (32 wt%). The char yield from acid washed wood samples was lower (about 6 wt%), and it was higher for raw wood and for wood samples impregnated with different cations (10-15%). The maximum gas yield was lower at approximate to 34 wt% for acid washed wood and much higher (approximate to 58 wt%) for the wood samples impregnated with potassium and sodium cations. These results, as expected, confirm the marked catalytic effects of cations on the post-pyrolysis cracking reactions of tar (the major product of wood pyrolysis) and the formation of char and gaseous products via cracking reactions of tar. In all these cases, sodium and potassium cations showed a stronger effect than calcium cations. The tar molecular weight was measured for wood samples impregnated with different cations. Tar molecular weight decreased with addition of cations to the wood particles, whereas it remained relatively constant with pyrolysis temperature. The tar molecular weight dropped from about M-w = 300 amu and M-n = 155 amu for raw and acid washed woods to about M-w = 190 amu and M-n = 100 amu for woods impregnated with potassium or sodium cations.