Cellulose decomposition without catalyst, liquefaction with Na2CO3, and gasification with Ni were studied in an autoclave and in a 50-nL microreactor (diamond anvil cell; DAC) coupled with optical and infrared microcopy in subcritical water up to 350 degreesC. Solid residue was the main product for noncatalytic decomposition of cellulose, which took place mostly under heterogeneous conditions at slow heating rates (0.18 degreesC/s). Homogeneous conditions could be achieved at a high heating rate of 2.2 degreesC/s, and solid residue was still the main product, but with different structure and reaction pathways. When Na2CO3 and Ni were used, however, oil and gas were the main products, with little residue being formed. In both catalytic cases, liquefaction and gasification seemed to occur in the aqueous phase after cellulose dissolution. Different reaction mechanisms for homogeneous and heterogeneous environments are proposed. It is expected that by using a high heating rate (e.g., 2.2 degreesC/s), a homogeneous phase can be achieved and more oil and gas can be produced to avoid solid residue formation.