Fouling in heat exchangers is a major economic problem for the crude oil industry. Despite significant research in this area, the fundamentals of the complex fouling process are not fully understood. There are many analytical techniques that are useful in the characterization of such deposits; however, each has its own limitations and drawbacks. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopic imaging has the advantage of being a nondestructive analytical technique with minimal sample preparation and, most importantly, is able to provide both chemical and spatial information about it sample. This paper introduces novel applications combining macro- and micro-ATR modes in FTIR imaging to characterize real deposits front a heat exchanger of a crude oil refinery. A lab-made aperture was utilized in macro-ATR imaging to correct the distortion of spectral bands that occurs for materials with a high refractive index, such as petroleum deposits. Using different ATR accessories for the macro (Golden Gate with a diamond crystal) and micro (germanium coupled to an infrared microscope) modes, FTIR imaging, with fields of view of 610 x 530 mu m(2) and 63 x 63 mu m(2). respectively, yields important information about the spatial distribution of different components in the deposits. Macro-ATR-FTIR imaging revealed clusters of chemically different compounds such as asphaltenes, carbonates. sulfates, sulfoxides, oxalates, and possibly coke. It was also demonstrated that with the enhanced spatial resolution of the micro-ATR approach, a more representative spectrum of the components can be achieved. Different forms of the oxalate functional group, one of which was barely at the detection limit of the macro-ATR approach, were also identified using the micro-ATR method. The sensitivity of the two modes of measurement with different spatial resolutions allows spatial and chemical information to be obtained and analyzed for domains of different sizes in studied samples.