The mass spectra of many rifamycins cannot be obtained by electron ionization (EI) owing to their thermal decomposition. When a laser beam is used to vaporize the sample through an optic fibre inserted in a hollow probe which reaches the sample cup, decomposition is minimized and the EI spectra show abundant molecular ions and fragments of structurally high diagnostic value. These ionic species are easily observed owing to the lack of chemical noise often present in soft ionization methods, such as direct liquid chemical ionization and fast atom bombardment.