This paper presents a conceptual model of the supply chain characteristics leading to supply chain complexity. This is combined with the change complexity of supply chain improvements, to reflect the complexity found in supply chain design decision-making when improving global supply chains. These two dimensions are used empirically, in the investigation of eight embedded cases of supply chain re-design, in a global OEM. Three contributions are made, improving the understanding of the link between supply chain design decision-making and supply chain complexity. First, the impact of different types of supply chain complexity on decision-making. Detail complexity leads to a higher need for resources for data collection and analysis, while dynamic complexity leads to challenges in predicting future performance. Second, the degree of change complexity is determining the potential supply chain complexity reduction. Third, a systematic bias resulting from low transparency on the marginal impact of increasing or decreasing supply chain complexity is proposed to lead to increasing supply chain complexity.