RESEARCH on microporous solids has focused largely on inorganic materials such as aluminosilicates (zeolites), aluminophosphates, pillared clays and other layered materials(1,2). An elusive goal has been the design of new materials with specific properties such as selective adsorption and catalytic activity. It would be very useful if the tools of molecular synthesis could be brought to bear on this problem. Here we report the design, based on a modular approach, and the crystal structure of an organic solid with large-diameter (about 9 Angstrom) extended channels. The channels are formed from planar, rigid macrocyclic building blocks. Onto the outer rim of the macrocycles are attached phenolic groups, which form hexagonally closest-packed two-dimensional hydrogen-bonded networks. Extended channels result from the stacking of these layers in a way that maintains registry between the macrocyclic cavities, and these channels are filled with solvent molecules. This approach potentially offers a simple means to exercise control over pore size and shape in the solid state.