The accuracy of instruments measuring the alveolar lung deposited surface area (LDSA) concentration based on unipolar diffusion charging has been assessed. Monoclisperse particles with sizes between 10 nm and 700 nm were used The results indicate that the LDSA concentration can be measured with at least +/- 30% accuracy for particle sizes between 20 nm and 400 nm. The LDSA concentrations of particles < 20 nm are overestimated and of particles >400 nm increasingly underestimated by the instruments. The LDSA concentration of 685 nm particles was on average underreported by a factor of approximately 2. LDSA concentrations measured with agglomerated particles were consistently higher than those measured with spherical particles of the same electrical mobility diameter. The accuracy was, however, still mostly within +/- 30% for a size range from 20 nm to 400 nm. Due to the strong and increasing deviation for large particles, the use of an appropriate preseprator is recommended when measuring LDSA concentrations. (C) 2015 Elsevier Ltd. All rights reserved.