While trust and opportunism were traditionally considered as the opposite ends of a continuum, when considered through the lens of paradox theory, these two could not only coexist, but also have a complementary effect on the achievement of sustainability in buyer-supplier relationships. To examine the combined effects of trust and opportunism, we apply a polynomial regression analysis and the response surface approach using survey data collected from 259 German firms. We find that the trust-opportunism paradox has different impacts on sustainability collaboration and evaluation. Sustainability collaboration thrives under high levels of trust and low levels of opportunism; here, firms can apply a resolution strategy to deal with the paradoxical tension. For sustainability evaluation, trust and opportunism jointly increase sustainability evaluation initially and then it decreases; in this case, a paradox acceptance strategy would be more appropriate. We also find these effects to differ between younger and mature relationships. In younger relationships equal levels of trust and opportunism result in the highest levels of sustainability collaboration. In mature relationships, the shadows of the past and the future seem to provide sufficient assurance to the buyer that sustainability collaboration can be sustained even in the presence of minor acts of opportunism.