CO oxidation an Pr(100) was investigated at reactant pressures of 8 Pa using ellip-somicroscopy for surface imaging to monitor spatiotemporal pattern formation on the catalyst surface and mass spectroscopy to monitor the integral reaction rate Three distinct temperature regions showing nonlinear phenomena were found. The first region, at 540 - 615 K, is characterized by macroscopic, regular reaction-rate oscillations with spatially uniform, instantaneous transitions between CO and oxygen covered states. Regular; periodic oscillations were nor observed on Pt(100) before. The second region, 480-540 K exhibited continuous dynamic pattern formation with no visible macroscopic kinetic oscillations Front speeds measured indicated a decrease in oxygen front velocity and an increase in CO front velocity with an increasing GO/O-2 partial pressure ratio. The third region, 390 - 430 K, is characterized by macroscopic rate oscillations, which continued for several oscillation periods and then ceased with the onset of pattern formulation.