Abstract: A mixture of urea, water and ammonium carbamate is formulated for use in the catalytic reduction of oxides of nitrogen in diesel exhaust. The mixtures may be formulated to optimize the amount of reductant in the mixture and the freezing point of the formulation. These formulations are especially useful in combination with Selective Catalytic Reduction systems and are well suited for use on heavy-duty trucks and heavy duty equipment used off-road. Some of these formations include between about 15.0 wt. % to about 40.0 wt. % urea; between about 15.0 wt. % to about 40.0 wt. % ammonium carbamate; and between about 40.0 wt. % to about 60.0 wt. % water. The formulation may be monitored for ammonia content and/or freezing point and the composition of the formulation may be adjusted to optimize the freezing point and ammonia content.