AbstractThe effect of the nature and alkyl chain length of zinc dialkyldithiophosphate (ZDDP) (R = C4 to C8) on its interactions with polyisobutenyl succinimide (PIBS) has been investigated using 31P‐NMR and infrared spectroscopy. 31P‐NMR spectra of ZDDP in the presence of PIBS showed both broad (35–850 Hz) and sharp (50–100 Hz) resonance signals due to different modes of interaction, mainly cluster formation and specific complexation, respectively. Single‐ended mono‐PIBS exhibit stronger interactions compared to the double‐ended bis‐PIBS. The proportion of sharp signals due to specific complexes in 31P‐NMR spectra of ZDDP—PIBS blends are in the order: C8 > C5 > C4+C8 > C6 = aryl ZDDP. The effect of PIBS concentration on the strength of complexes has also been studied.