A Fourier transform ion cyclotron resonance (FTICR) mass spectrometer has been used to trap individual multiply charged ions of several high molecular weight polymers, including poly(ethylene oxide), sodium poly(styrene sulfonate), and the protein bovine serum albumin. Detection of these ions is performed with the nondestructive method distinctive of FTICR, which also allows remeasurement of the same ion or ion population over several hours. For the determination of the charge states (and hence the masses) of individual ions, a new scheme was developed on the basis of the observation of the stepwise mit shifts that result from charge exchange reactions or adduction of a substance of known mass. A novel technique for mass determination of individual ions has been made possible with the observation of cyclotron frequency shifts during the time-domain acquisition period. This time-resolved ion correlation (TRIC) technique allows reactant and product ions to be correlated with confidence and provides the basis for simultaneously studying a moderate number of ions. In this work, a range of observations related to the detection and measurement of Individual ions is presented, as are examples of mass determinations of individual ions performed by utilizing the TRIC technique. Results are presented that show the measurement of poly(ethylene glycol) (PEG) individual ions of more than 5 MDa with more than 2500 net charges and measurement of ions as small as albumin (66 kDa) with as few as 30 charges. Other results illustrate that unexpected behavior can be observed for individual ions that would not be apparent in large ion populations.