Detailed characterization of zeolites SSZ-26 and SSZ-33 is presented and includes data from adsorption experiments, X-ray powder diffraction (XRD), electron diffraction (ED), and high-resolution electron microscopy (HREM). Simulated XRD patterns and HREM images of the proposed structures agree very well with all the experimental data. These zeolites are the first synthetic high-silica molecular sieves to contain a three-dimensional pore system comprised of intersecting 10- and 12-ring ports. The structures of SSZ-26 and SSZ-33 are compared to the structures of known zeolites, and the relationships between the geometry of the structure-directing agents used in the synthesis of SSZ-26 and the geometry of the pores are analyzed using energy minimization calculations. Several hypothetical zeolite structures are derived based on the proposed models for SSZ-26 and SSZ-33. SSZ-26 and SSZ-33 are the first examples of molecular sieves whose multidimensional pore systems have been formed by the purposeful design of their structure-directing agents.