The effects of high boiling point fuel additives on deposits were investigated in a commercial turbocharged direct injection gasoline engine. It is known that high boiling point substances have a negative effect on deposits. The distillation end points of blended fuels containing these additives may be approximately 15°C higher than the base fuel (end point: 175°C). Three additives with boiling points between 190 and 196°C were examined: 4-tert-Butyltoluene (TBT), N-Methyl Aniline (NMA), and 2-Methyl-1,5-pentanediamine (MPD). Aromatics and anilines, which may be added to gasoline to increase its octane number, might have a negative effect on deposits. TBT has a benzene ring. NMA has a benzene ring and an amino group. MPD, which has no benzene ring and two amino groups, was selected for comparison with the former two additives. The base gasoline was a Toyota in-house premium grade test gasoline with properties in the range defined by the Japanese Industrial Standards (JIS) (RON: approximately 100) with no detergent content. Test gasolines were prepared by blending the base gasoline with 10% of each additive by volume. The concentration of the additives was set to 10% to accelerate deposit formation. The engine operating conditions for examining deposit formation were an engine speed of 1,600 rpm and medium load. Deposit formation was examined over a period of 30 hours, after which the fuel consumption was approximately 200 L. It was found that amino group additives caused large increases in deposits. Compared to the base gasoline, the piston top deposits were about twice as thick with the TBT blend and about four times as thick with the NMA blend. The MPD blend caused compression leakage after fuel consumption of 10 L because the piston rings stuck to the grooves. Chemical analysis of the deposit formation mechanism suggests that deposits were formed by high boiling point polar substances that penetrated into the quenching zone near the combustion chamber surfaces, and then oxidized, polymerized, or carbonized, and adhered to the surfaces.