There seems to be no method available to measure all parameters of interests in multiphase #ow systems with high solids concentration and with reasonable accuracy. Parameters include, local particle velocity vectors, solids volume fractions and bubble rise velocity, size, frequency and volume fraction. In this study a novel method based on a "ber optical technique and tracer particles has been developed for simultaneous measurements of the mentioned local #ow properties in highly concentrated multiphase #ow systems such as gas}solid #uidized bed reactors. A particle present in the measuring volume in front of the probe is marked with a #uorescent dye. A light source illuminates the particles and the detecting "bres receive re#ected light from uncoated particles and #uorescent light from the tracer particle. Using optical "lters, the #uorescent light can be distinguished and together with a small fraction of background light from uncoated particles can be used for the determination of local #ow properties. The method gives the possibility for simultaneous measurement of the local movement of a single tracer particle, local bubble properties and the local solids volume fractions in di!erent positions in the bed.