• Electron transfer at modified electrodes;
• Classical and statistical thermodynamics of electrified interfaces;
• Structural phase transitions in minerals of the calcite group;
• Properties of molecules in excited states (dipole moments, acidity constants and other thermodynamic properties, heterogeneous electron transfer). 

The major research interest of Prof. Willem Mulder's group involves the application of thermodynamics to various areas, such as electrochemistry, with specific focus on surface and colloid chemistry. The objective of the research is to contribute to a deeper understanding of the electrochemical behaviour of modified electrodes which are used in biomimetics and could ultimately lead to the design of improved electrochemical sensors. These studies aim to elucidate the mechanisms of charge transfer reactions and to shed light on the structure/property relationships between molecules and materials. 
Additionally, the group is currently involved in investigating some mathematical aspects of phylogenetic trees. These are objects that describe the historical relationships among living organisms, based on similarities in traits and genetic makeup of extant species. This work will aid in the reconstruction of the “tree of life”, one of the main goals of evolutionary biology. 
Research is also under way in the area of mineralogy, in particular the structural phase transitions in minerals of the calcite group; the kinetics of glucose uptake by t-cells as part of an immunological study related to HIV/AIDS research; and the mathematical ecology of insect populations. 
Electrochemistry has useful application in energy storage and power generation (fuel cells, photovoltaics); colloid chemistry can be applied in industry (nano-science, oil winning, cosmetics, foodstuffs, paper-making, inks, soaps and detergents), chemical catalysis, geology (soil science) and the life sciences.
 Following a curiosity-driven modus operandi, the group uses theoretical research methods to develop a mathematical model for a physical/chemical system. Such models are developed from raw data provided by experimentalists. The predictions of the models are then worked out and experiments conducted to verify them.
 Prof. Mulder has published 48 journal articles on his research and is currently working on aspects of electrochemical kinetics, and the mathematical underpinnings of phylogenetics and insect population dynamics.