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An experimental photochemistry study involving gas‐ and solid‐phase amino acids (glycine, DL‐valine, DL‐proline) and nucleobases (adenine and uracil) under soft X‐rays was performed. The aim was to test the molecular stabilities of essential biomolecules against ionizing photon fields inside dense molecular clouds and protostellar discs analogs. In these environments, the main energy sources are the cosmic rays and soft X‐rays. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing 150‐eV photons.  In situ  sample analysis was performed by time‐of‐flight mass spectrometer (TOF‐MS) and Fourier transform infrared (FTIR) spectrometer, for gas‐ and solid‐phase analysis, respectively. The half‐life of solid‐phase amino acids, assumed to be present at grain mantles, is at least  3 × 10 5   and  3 × 10 8   yr inside dense molecular clouds and protoplanetary discs, respectively. We estimate that for gas‐phase compounds these values increase 1 order of magnitude since the dissociation cross‐section of glycine is lower in gas phase than in solid phase for the same photon energy. The half‐life of solid‐phase nucleobases is about 2–3 orders of magnitude longer than found for amino acids. The results indicate that nucleobases are much more resistant to ionizing radiation than amino acids. We consider these implications for the survival and transfer of biomolecules in space environments.

Photostability of gas‐ and solid‐phase biomolecules within dense molecular clouds due to soft X‐rays