The exact masses of bastadins, cyclic peptides from marine sponges Ianthella basta, are determined using matrix-assisted laser desorption ionization (MALDI) coupled to a Fourier transform mass spectrometer. Two known compounds were mixed with the unknown to serve as internal calibrants. The mass of the calibrants bracketed the mass of the unknown compound. With this method, exact masses were obtained to within 5 ppm for single determinations, and less than 3 ppm for multiple determinations, allowing the derivation of elemental composition. This method is viable for routinely obtaining the exact masses of new compounds with MALDI.