This paper is Part 1 in a series of two describing probe measurements of deposit build-up and removal (shedding) in a 350 MWth suspension boiler, firing straw and wood. The influence of fuel type (straw share in wood), probe exposure time, probe surface temperature (500, 550, and 600 °C), and flue gas temperature (600–1050 °C) on ash deposit formation rate has been investigated. Investigations of deposit formation rate were made by use of an advanced online deposit probe that allowed nearly continuous measurement of the deposited mass. Two different measures of deposit formation rate are used in the analysis of the data. The first is the integral deposit formation rate (IDF-rate) found by dividing the integral mass change over integral time intervals (of order several hours) by the time interval. The IDF-rate is similar to deposit formation rates based on total deposit mass uptake divided by probe exposure time reported in previous full-scale investigations, but it is a relatively crude measure that includes all deposit shedding in addition to actual deposit formation. To remove major shedding events from the determination of deposition rates a second measure, the derivative-based deposit formation rate (DDF-rate), was devised. This was determined by averaging the deposit mass uptake signals over short time intervals (on the order of minutes), calculating the local values of the time derivative of the mass uptake, removing large negative values signifying major shedding events, and finally time smoothing the derivatives to remove excessive noise. The DDF-rate was influenced by flue gas temperature and straw share, while changes in probe surface temperature had no significant influence. The IDF-rate, qualitatively related to the ratio between the time-integrated DDF-rate and the integration time, followed the same trends. Ash transformation was investigated by bulk ash analysis of the fuel, fly, and bottom ash during straw and/or wood suspension firing. Bulk ash analysis of fly ashes showed that the contents of volatile elements (K, Cl, S) were slightly greater than in the fuel ash, while Ca and Si remained either in the same proportion or were slightly reduced. It was also found that, with an increase in fuel ash K/Si molar ratio, the concentration of the volatile elements, K, Cl, and, to some extent, S, increased in the fly ash. The bottom ash was dominated by Si and Ca, with almost no S and Cl, possibly as a result of the high volatility of S and Cl during combustion at higher temperatures.