Let the cost price of each pen be \( x \). Therefore, the total cost price of 16 pens is \( 16x \).
The selling price of each pen is \( \frac{16x}{12} = \frac{4x}{3} \). The total selling price of 12 pens is:
\[
12 \times \frac{4x}{3} = 16x.
\]
So, the selling price is greater than the cost price. The profit made on selling one pen is:
\[
\frac{4x}{3} - x = \frac{x}{3}.
\]
Thus, the percentage profit is:
\[
\frac{\frac{x}{3}}{x} \times 100 = 33 \frac{1}{3} \%.
\]