Kinetic studies were conducted to elucidate the effects of diffusional limitations at particle scale during thermogravimetry analysis (TGA) char reactivity tests. The main objective was to identify the limiting phenomena that may occur during biomass char gasification experiments with CO2. The char used was wood matter from pressed-oil stone (WPOS) (this is also called orujillo). The gasification rates of WPOS were measured at four different particle sizes (0.06, 0.9, 1.2, and 2.1 mm), three CO2 partial pressures (0.20, 0.35, and 0.50 bar), and four temperatures (800, 850, 900, and 950 degrees C). Literature on single-particle studies and diffusional effects that occur in char gasification kinetic experiments were critically surveyed. We found neither general criteria for a kinetic model to be selected nor a quantitative consensus about the impact of variables such as particle size on the diffusional interferences. Our own experiments indicate the extraordinary importance of the diffusional effects at high temperature and large particle size, i.e., a large Thiele modulus. The CO2 partial pressure was seen to have a secondary role, compared to temperature and particle size, within the range analyzed. The results presented in this study reveal that biomass-derived char particles inside industrial gasifiers may be limited by diffusional effects. Consequently, suitable kinetic-particle models that capture plausible physical limitations should be used when simulating such gasifiers. In the second part of this work, which is given in a companion paper [Energy Fuels 2006, 20, xxxx], a simple particle model is developed and validated with the experimental results reported in the present article.