When criminal tax proceedings are followed by civil tax proceedings, the legal doctrine of collateral estoppel may apply. This doctrine provides that an issue necessarily decided in a previous proceeding (the first proceeding) will determine the issue in a subsequent proceeding (the second proceeding) but only as to matters in the second proceeding that were actually presented and determined in the first proceeding.
A conviction for criminal tax evasion collaterally estops the taxpayer from contesting the existence of tax fraud for purposes of the civil tax fraud penalty (i.e. 75% of the unpaid tax) because a finding of criminal tax fraud (beyond a reasonable doubt) establishes proof of civil tax fraud (by clear and convincing evidence).
Acquittal of criminal tax evasion does not collaterally estop the government from proving civil tax fraud (by clear and convincing evidence). The criminal acquittal may establish that proof of fraud did not exist beyond a reasonable doubt, but that does not mean that proof of fraud by clear and convincing evidence does not exist.