Devolatilization of different types of polymer melts was studied experimentally and theoretically. The experimental work was carried out in a "falling-strand"-type apparatus build especially for this study. Polymer samples after devolatilization we examined by scanning electron microscopy. The micrographs obtained showed that strong nucleation of bubble nuclei occurs first in the vicinity of existing primary microbubbles. These bubbler nuclei (secondary nuclei) form blisters (microbubbles) growing under appropriate conditions. The growth is controlled by momentum transfer and diffusion. The microbubbles then coalesce leading to formation of large voids and foaming of the polymer strands. The key parameter, the critical bubble radius triggering the process of growth, was assessed from the micrographs. The time dependence of the concentration of the residual styrene (dissolved gas) in polystyrene melt was determined by gas chromatography. A theoretical model of stress-enhanced secondary nucleation in polymer melts during their devolatilization was developed. It shows that nucleation proceeds due to the growth of initially existing gas bubbles, when a sample is exposed to vacuum. In the vicinity of the bubbles homogeneous nucleation proceeds at an accelerated rate, because it is strongly enhanced by mechanical degradation of polymer macromolecules. the rate of devolatilization was calculated and compared with the experimental data. Theoretical results agreed with the experimental data rather good, although no adjustable parameters characteristic of some of the existing nucleation models were involved.