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Electronic structure of multiwall carbon nanotubes has been studied by X-ray emission and photoelectron spectroscopy. The measurements were performed for two samples synthesized by different methods such as arc-discharge graphite evaporation and catalytic decomposition of hydrocarbons on supported catalysts. X-ray spectra of the former sample were shown to be similar to those of the highly oriented pyrolytic graphite, while the C Kα spectrum of catalytic nanotubes exhibited an enhanced density of π occupied states and the C 1s line of sample expanded toward both lower and higher binding energy regions. The models incorporating pentagon−heptagon pair and sp3-hybridized atoms have been calculated by AM1 quantum-chemical method to reveal the influence of defects on the electronic structure of carbon nanotube. Carbon Kα and 1s lines were found to be practically unaffected by inserting pentagon and heptagon into the hexagonal network on diametrically opposite sides of tube. However, the changes observed in the X-ray spectra of catalytic carbon nanotubes may result from adjacent pentagon−heptagon pairs and interlayer linkages with more noticeable effect from the latter defect kind.