The etch mechanism of porous SiOCH-based low-k films by F atoms is studied. Five types of ultra-low-k (ULK) SiOCH films with k-values from 1.8 to 2.5 are exposed to F atoms in the far downstream of an SF6 inductively coupled plasma discharge. The evolution of etching with an F dose was studied using various techniques of surface and material analysis such as FTIR, XPS, EDS and SE. It is revealed that the etch mechanism is connected with surface fluorination and formation of –CHxFy species on the surface due to H abstraction by F atoms from –CH3 groups. It is shown that the etching includes two phases. The first one is observed at the low F doses and is connected with chemical modification and etching of walls in the topmost pores, which finishes when the walls are fully etched. At the same time, the additional etching in the underlying pores due to F penetration forms the etch depth profile, after that the second etching phase starts. This phase is characterized by the higher etch rate due to the propagation of the etch depth profile further into the film. The preliminary treatment of pore walls inside porous channels effectively accelerates etching many times compared to non-porous material. The acceleration depends on the modification depth, which in turn is a function of pore structure and interconnectivity as well as the F atom reaction mechanism. The combined random walk (Monte-Carlo) & kinetics model developed to describe F penetration inside SiOCH films together with reactions of F atoms leading to –CHxFy depletion and opening SiOx bonds for F access allowed relating the increased etch rates with increasing the total number of F atom collisions inside interconnected pores. The etch mechanism of SiOCH films is found in many respects to be similar to the SiO2 etch mechanism on the elementary level, but as whole it is ruled by the SiOCH structure: porosity degree, pore size, pore interconnectivity as well as structural features of SiOx bonds.