This paper is concerned with the on-line determination of alkali traces in solid fuel combustion using simultaneous surface ionisation (SI) and excimer laser induced fragmentation fluorescence (ELIF). Both methods measure in real-time but are complementary in that ELIF can discriminate towards gas phase alkali species, whereas SI detects alkali both in the gas phase and on aerosol particles. Measurements were made in the flue gas of a 10 MW PCFB combustor at 10 bar total pressure, whereby the measuring equipment was mounted onto the flue gas pipe downstream of the hot gas filter. The combustor was fed with a bituminous coal and operated in the temperature range 850-900 degreesC. Results show that the general response of the measured signals to parameters such as fuel load and to bed and flue gas temperatures was very similar for the two methods and reflected variations in these parameters. Concentrations determined by ELIF ranged from 6 to 63 ppb (Na) and 1 to 54 ppb (K), depending on conditions. In contrast, concentrations obtained by the SI technique were a factor of 1 - 10 higher than for ELIF, depending on the flue gas temperature and the total alkali (gas + particle phase) present in the flue gas. In this way it can be shown that information about the physical state of the alkali species can be obtained by direct comparison of the two methods. (C) 2002 Elsevier Science Ltd. All rights reserved.