The pyrolysis of wood pellets was investigated using a single-mode microwave cavity, and the dielectric properties of the wood were Measured at temperatures up to 750 degrees C. Below 600 degrees C, the only microwave-absorbing phase within wood is water. This study has shown categorically that microwave pyrolysis can be achieved without the use of carbon-rich dopants and that the heating of water alone call be used to induce pyrolysis of wood. A number of potential mechanisms are discussed that relate to the power density within the heated material. The yield of bio-oil and biogas is a function of the heating rate and power density, and for the samples used in this study, a threshold power density of 5.0 x 10(8) W/m(3) was found, below which microwave pyrolysis did not occur. The results and hypotheses presented in this article represent the first steps in understanding the fundamental mechanisms of microwave pyrolysis.