Secondary ion mass spectrometry (SIMS) was used to measure the Er, Al and P distributions across (perpendicular to) and along (parallel to) the axis of Er3+-doped silica based preforms and fibers prepared by a solution doping technique. Specially prepared low-melting silicate glass standards containing known concentrations of Er were used to calibrate the spectrometer to obtain a semi-quantitative measurement of the Er concentration. It was found that the Er concentration can vary along a 4-cm section of the preform by as much as a factor of two. The Al concentration also changed along the length of the preform, but to a smaller degree. Finally, instrumental limitations for measuring the Er, Al and P distributions in the optical fiber samples were determined.