Polyisobutenylsuccinic anhydride (PIBSA) has been analyzed using negative ion electrospray ionization mass spectrometry (ESI-MS). This mild technique gives the molecular ion distribution for the PIBSA mono-succinic acid polymer as the major product. Unexpectedly, small amounts of the PIBSA bis-succinic acid polymer were also observed. This can form a basis for a method to determine the succinic ratio of samples of PIBSA. Commercially available samples of PIBSA made from BF3-catalyzed poly(isobutylene) and AlCl3-catalyzed poly(isobutylene) were compared. While the PIBSA made from BF3-catalyzed poly(isobutylene) consisted of polymers separated by 56 Da, the PIBSA made from AlCl3-catalyzed poly(isobutylene) consisted of polymers separated by 14 Da. We attribute this to different molecular weight distributions of the starting poly(isobutylene). Poly(isobutylene) from AlCl3 catalysis is postulated to form via a mechanism that consists of carbenium ion rearrangements and P scission reactions at the active site.