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We examined the vacancy trapping proficiency of Sn and P atoms in germanium using positron annihilation spectroscopy measurements, sensitive to the open-volume defects. Epitaxial Ge 1 − x Sn x films were grown by chemical vapor deposition with different P concentrations in the ∼ 3.0 × 10 19 – 1.5 × 10 20 cm − 3 range. We corroborate our findings with first principles simulations. Codoping of Ge with a Sn concentration of up to 9% is not an efficient method to suppress the free vacancy concentration and the formation of larger phosphorus–vacancy complexes. Experimental results confirm an increase in the number of P atoms around the monovacancy with P-doping, leading to dopant deactivation in epitaxial germanium–tin layers with similar Sn content. Vice versa, no impact on the improvement of maximum achieved P activation in Ge with increasing Sn-doping has been observed. Theoretical calculations also confirm that P n -V (vacancy) complexes are energetically more stable than the corresponding Sn m P n -V and Sn m -V defect structures with the same number of alien atoms (Sn or P) around the monovacancy. The strong attraction of vacancies to the phosphorus atoms remains the dominant dopant deactivation mechanism in Ge as well as in Ge 1 − x Sn x. [ABSTRACT FROM AUTHOR]