The state of aggregation among virions suspended in water was measured and quantitatively characterized by several methods developed in this research. All unpurified virus suspensions contained some aggregates and all of them contained half or more singles. Stable purified suspensions of single virions without detectable aggregation were prepared and used for comparison of inactivation kinetics of several viruses under a variety of conditions. All of the viruses tested tended to aggregate at acid pH, but the pH below which aggregation began was quite different for different viruses. All the viruses tended to aggregate at low ionic strength, but the kind of salt or buffer present strongly influenced the rate of aggregation at a given pH and temperature.