A four-cylinder engine with a slice between the head and the block carrying instrumented plugs has been used to study the growth of combustion chamber deposits and knock. Deposit thicknesses vary substantially at different locations, the thickness generally being greatest at the coolest surfaces. If a dirty engine is run on a low-boiling-point fuel such as a primary reference fuel, deposits are removed and octane requirement is reduced rapidly. Of the head deposits, those in the cooler squish region where the end gas is likely to be situated affect knock more than the deposits in the hotter regions. Different fuel additives have different effects on deposits in different areas. For instance, an additive might cause a substantial increase in deposit thickness in the hotter areas and a slight increase in total deposit weight but can control deposits in the cooler squish regions and so reduce octane requirement increase (ORI) compared to the base fuel alone. Different fuels respond to the same deposits to different extents in terms of knock. Knock ratings can depend on the fuel the deposits have been exposed to just prior to the rating. Low octane, low-boiling-point fuels appear to make the engine more prone to knock probably by facilitating the release of pro-knock species trapped in the deposits. This “chemical” ORI can be alleviated by running the engine on a full boiling range gasoline for a short time.