We study diffusive electron-electron interaction correction to conductivity by analyzing simultaneously ρ_xx and ρ_xy for disordered 2D electron systems in Si in tilted magnetic field. Tilting the field is shown to be a straightforward tool to disentangle spin and orbital effects. In particular, by changing the tilt angle we prove experimentally that in the field range gμ_BB>k_BT the correction depends on modulus of magnetic field rather than on its direction, which is expected for a system

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... isotropic g-factor. In the high-field limit the correction behaves as (B), as expected theoretically (Lee, Ramakrishnan, Phys. Rev. B 26, 4009 (1982)). Our data prove that the diffusive electron-electron interaction correction to conductivity is not solely responsible for the huge and temperature dependent magnetoresistance in parallel field, typically observed in Si-MOSFETs.