In this chapter, a method for the quantitative determination of amyloid conversion by electrospray mass spectrometry is presented. Mass spectrometry is typically used for the purpose of measuring the mass of unknowns. However, the judicious selection of an internal standard permits the quantitative determination of protein concentration. For amyloid formation, this is particularly useful in circumstances where either the protein under study is in limited abundance, or separation of the precursor from other protein factors is impractical or undesirable. For the measurement of amyloid formation, internal standards are typically mass distinct variants of the amyloid precursor. In addition, the extreme stability of amyloid fibers permits assessment of residual precursor concentration with or without separation of fibers from unreacted precursor. Lastly, by using internal standards which are not amyloidogenic and do not interfere with fiber formation, electrospray mass spectrometry permits quantitative measurement of fiber formation in real-time and with small (pmoles) quantities of protein.