The crosstalk in CMOS photodiodes has been measured, at two wavelengths of 543 and 633 nm, by an experimental structure containing several types of photodiodes with varying dimensions. The role of the design of the junction in reducing crosstalk is studied. The measurements indicate that to reduce crosstalk it is essential to optically shield the gap between junctions and to reverse bias the adjacent junctions. Crosstalk is significantly reduced in double-junction photodiodes, but at the cost of lower quantum efficiency. The results indicate that with properly designed layout, the crosstalk may be small. However, there is a tradeoff between small crosstalk, reduced fill factor and quantum efficiency. (C) 2002 Elsevier Science Ltd. All rights reserved.